Last Updated: 19 August 2001

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**ADAPTIVE PROBING:**
A management device, characterized by exploration of diverse strategies,
used to improve information about alternatives.

**AGGREGATION:** Lumping
diverse objects together in a single category.

**AMPLIFY**: A system component or process that increases
the magnitude of fluctuations or changes of a system variable. See *DAMP*.

**ASYMPTOTE**:

**ASYMPTOTIC**: An asymptotic function gradually approaches
a constant value (the ASYMPTOTE); after some period time the function is
arbitrarily close to the constant.

**BUDGET**:

**CELLULAR AUTOMATA:**
Simplified mathematical models of spatial interactions, in which sites
or cells on a landscape are assigned a particular state, which then changes
stepwise according to specific rules conditioned on the states of neighboring
cells.

**CHAOS:** Complex dynamical behavior
characterized by a lack of dominant periodicity and great sensitivity to
initial conditions.

**COMPUTER MODEL (also called NUMERICAL
MODEL)**: A construct of mathematical and logical
statements that describe a complex system in quantitative terms with the
help of a computer; a carefully constructed, but sharply limited **simulation**
of nature in a computer. See also **DEMONSTRATION**,
**CONCEPTUAL
MODEL**.

**CONCEPTUAL MODEL**:
A mental image of an object, system, or process.

**CONSERVATION LAW**: A statement describing a process
wherein matter or energy or both are transformed, transported or stored
without loss.

**CONSERVATION
OF ANGULAR MOMENTUM:**

**CONSERVATION OF MATTER**: Matter may be neither created
nor destroyed. A piece of matter may under go a *CHANGE OF PHASE*
or may react chemically either within itself or with other matter, so it
may not be recognizable in the same form, but the amount of matter does
not change.

**CONSERVATION OF ENERGY**: Energy may be neither created
nor destroyed. An amount of energy may under go a transformation or may
be transported to another location, but the total amount of energy does
not change. Within a *CLOSED SYSTEM*, energy may be transformed from
one form to another, but the total amount cannot change. In an *OPEN
SYSTEM*, there are exchanges of energy with the surrounding environment.
During these exchanges, however, the change in total system energy must
exactly equal the energy exchanged. Conservation of energy is the *First
Law Of Thermodynamics*.

**CONSTANT**: A quantity that has a fixed mathematical
value within a model, such as pi or the Earth's radius. It may appear explicitly
in equations, logical statements, or as an initial value, or it may be
a named parameter that the model uses.

**CONSTRAINT**: A restriction
to the behavior of a variable. Constraints are similar to *LIMITS*,
but usually act over a broader range of variable values.

**CONTROLLED EXPERIMENT**:
A form of scientific investigation in which one variable, termed the independent
or control variable, is manipulated to reveal the effect on another variable,
termed the dependent or responding variable, while all other variables
in the system are held fixed. See also *SYSTEMATIC OBSERVATION*,*
STATISTICAL EXPLANATION*.

**CORRELATION:** A statistical
estimate of the relatedness of two events. See also **UNCORRELATED**.

**CORRELATION LENGTH:**
The distance in space or the period in time beyond which events are **UNCORRELATED**.

**COUPLED SYSTEMS**:
Two systems are coupled if information from one is provided to, and influences
the behavior of, the other. The information being passed is termed the
*INTERCONNECTION*.

**CRITICALITY:** The condition
describing the transition between qualitatively different states, such
as solid/liquid or liquid/gas.

**CYBERNETIC:** Relating to
a control process.

**CYCLE**: A sequence of processes
through which a system variable (or flow) ultimately returns to the reservoir
from which it came. Sometimes called a *closed loop* in the system.

**DAMP**: A system component or process that acts to
decrease the extent of fluctuations or changes in a system variable is
said to damp the variable. See *AMPLIFY*.

**DATA**: A group of facts, often in numerical form,
describing the outcome of an experiment or the results of a systematic
observations. Data are *processed*, *filtered*, *analyzed*,
and *interpreted* to yield *INFORMATION*.

**DEMONSTRATION:** See
also **SIMULATION**.

**DISSIPATION:**

**DOUBLING TIME**: The time required for a function
or variable to increase its value by a factor of two.

**DRIVER FUNCTION**: See *FORCING FUNCTION*.

**DYNAMIC EQUILIBRIUM**:
The state of system wherein the output continually changes but remains
within fairly narrow bounds. The output is characterized by a mean value
and a bounded range around that mean value. Example: water liquid in balance
with the vapor, where the number of molecules leaving the liquid at any
moment is approximately (but probably not exactly) equal to the number
leaving the vapor for the liquid. But over any interval of time, these
two quantities are equal. We say that condensation and evaporation are
in dynamic equilibrium at saturation.

**DYNAMIC PARAMETER**: A value provided to a system
that may change with time either in a prescribed manner or in response
to the changing state of the system.

**e-FOLDING**: An exponential function changes by the
factor e or 1/e when the exponent of e changes by ~1.0. The time required
for the exponent to change, or the distance over which it changes, by ~
is called, respectively, the e-folding time or the e-folding distance.
See *EXPONENTIAL*.

**ENERGY**: The capacity to be active or to do work.

**ENERGY BUDGET**:

**ENERGY CONSERVATION**: See *CONSERVATION OF ENERGY*.

**ENTHALPY**:

**ENTROPY**: The scientific measure of the disorder
in a system; the greater the disorder, the greater the entropy. See *SECOND
LAW OF THERMODYNAMICS*.

**EQUATION**: A mathematical statement in which equal
values (or the mathematical statements producing the values) appear to
the right and left of an equal sign. In system modeling and programming,
an equation is an action by which the left-hand side is set equal to the
value produced by the evaluation of the right-hand side. The two sides
are equal after the action is taken but may not have been equal prior to
the action.

**EQUILIBRIUM**:

**EXPONENTIAL FUNCTION**:
A mathematical function, y = eax (here "a" is a CONSTANT and "e" is a constant
equal to 2.718280), in which y asymptotically approaches 1.0 as ax approaches
zero (*i.e.*, for ax << 1.0) and y increases without an upper
limit as ax increases far beyond 1.0 (*i.e.*, for ax >> 1.0).

**FEEDBACK**: When information
on a system's behavior is used by the system to modify its behavior, the
process of transferring the information across the system is called *feedback*.
This information is passed back through the system to the input via a *feedback
loop*. **Feedback loops can be tight (short,
well defined, rapidly responding) or loose (long, ill-defined, slow
responding).**

With** negative
feedback (-)**, as the Output
increases (decreases), the information passed back via the feedback loop
is subtracted from the Input, causing the inflow to the Reservoir to decrease
(increase). With negative feedback, the modification of the Input is in
the direction opposite to the behavior; in general, this constrains the
system and leads to stability.

**FLOW:** The movement of energy,
material, or information from one place to another.

**FLUX**: The rate at which a variable
enters or leaves a reservoir. As termed, a *flow*.

**FLUCTUATION**: Variations
in the value of a variable, usually around the variable's locally averaged
value.

**FLUSHING RATE:** The
rate which liquids that enter a region are removed from it.

**FORCING FUNCTION**:
A parameter that controls the behavior of a system and makes its behavior
regular and predictable. Also called the *DRIVER FUNCTION*.

**FRACTAL:** A self-similar structure
whose geometrical and topographical features are recapitulated in miniature
on finer and finer scales.

**FUNCTION:** A mathematical
relationship between variables. See also FUNCTION in an ecological sense.

**GLOBAL SCALE**:

**HEAT ENGINE:** A mechanism
that converts some of the energy present in a high temperature fluid into
work. It is composed of four components:

A working fluid;

A work to be done; and

A low-temperature energy sink.

**HOMEORRHESIS**: Regulation
of system property around a changing **set point**.

**INFORMATION**:

**INITIAL CONDITIONS**: The values assigned to all
system variables at the "start time", the time or date when computations
are to begin.

**INSTABILITY**:

**INTERACTION**: The relation among objects that do
something to one another. *Evidence of interaction* is an observable
effect that is interpreted as arising from the interaction of certain objects.
For example, ocean tides on Earth are evidence of interaction between the
Moon and ocean waters.

**INTERCONNECTION**: The information provided from
one system to another, or from one part of a system to another part, that
influences the behavior of the receiving system or part.

**INTERFACE**:

**ISOLATED SYSTEM**:
A system that has no significant interactions with other systems or with
the rest of the universe.

**MATTER:**

**MATTER CONSERVATION:** See CONSERVATION OF MATTER.

**MODEL**:

**NEGATIVE FEEDBACK**:
**Feedback**
that tends to counteract a process. See also **POSITIVE
FEEDBACK**.

**NONLINEAR**: Disproportionate
in cause and effect.

**NORMAL DISTRIBUTION:**
A statistical distribution of a specific symmetric nature, representing
the outcomes of a large number of basic random processes.

**OPTIMALITY:** Condition
of being best.

**OPTIMIZATION:** The search
for the best solution among alternatives, or the extreme value of a variable
or a function.

**PATH DEPENDENCY: **The
dependence of the outcome of a dynamical process on historical events.

**PATTERN FORMATION:**
The emergence of large-scale patterns from local processes and interactions.

**PERCOLATION:** Mathematical
theory of spread on cellular landscapes.

**PERIODIC:** Characterized
by repetition of patterns in fixed intervals of space or time.

**PHASE TRANSITION:**
Transformation of a substance or a system from one form to a qualitatively
different one.

**POSITIVE FEEDBACK**:
**Feedback**
that tends to reinforce a process. See also **NEGATIVE
FEEDBACK**.

**POWER LAW:** A relationship
between two variables such that one is proportional to a power of the other.

**PREDICTION**: A statement
foretelling the possible outcome(s) of an event, process, or experiment.
In meteorology, a prediction is also called a *forecast*. A prediction
is based on observations, experience, and scientific reasoning. A *guess*,
on the other hand, is based on conjecture (speculation), chance, and intuition.

**PRISONER'S DILEMA:**
A prototypical game theory problem involving two prisoners held in isolation;
illustrative of the difficulties in achieving cooperative behavior.

**RECIPROCAL ALTRUISM:**
Mutually beneficial behavior in which an individual helps another because
of expected reciprocal behavior.

RECOMBINATION: The genetic reshuffling of material with an organism due to the crossing over of chromosomes and reassortment of genes.

**RESIDENCE TIME**: The
average time an element of material spends in a reservoir; a characteristic
time for a reservoir.

**RESILENCY:** The capability
to resist qualitative shifts in behavior or characteristics.

**(TIME, SPACE) SCALE: Characteristic
extent (in time or space).**

**SCHEMA (pl. SCHEMATA): Building
block for discovering rules; an internal model of the real world.**

**SELF-ORGANIZATION:
The development of macro-scale structure and functioning on the basis of
local organization.**

**SELF-ORGANIZED
CRITICALITY: The tendency of large complex systems to self-organize to
a self-maintaining critical state.**

**SIMPLEX METHOD:** A
method developed by George Bernard Dantzig for solving linear program problems.

**SIMULATED ANNEALING:**
An optimization method that simulates the physical process of annealing
by allowing the occasional acceptance of less attractive solutions or values.

**SIMULATION:** See also **DEMONSTRATION**.

**SIMULATOR:** A model -- physical
or numerical -- that replicates some process. For example, SORTIE is a
forest growth simulator developed by Stephen Pacala and his colleagues.

**STABILITY**: The tendency
to return to normative behavior, such as an equilibrium or a limit cycle
in a mathematical model.

**STOCHASTIC:** Governed by
random chance, and so unpredictable.

**STOCHASTIC
CELLULAR AUTOMATA:** **Cellular automata**
in which rates are **stochastic**.

**STOCHASTIC OPTIMIZATION:**
A class of mathematical optimization techniques that rely in part on the
random generation of new variants of existing forms.

**SUBSYSTEM**: A system that
is entirely included in another system.

**SYSTEM**: A group of related
components or **subsystems** that form a whole.
Systems are often selected because the components interact or have the
ability to do so. Systems may be **open**
or **closed**, linear
or **non-linear**, stable
or unstable.

**THERMODYNAMICS**: Describes
the "direction" of a process.

**TIT FOR TAT:** A strategy
for cooperative behavior in which an individual, at each stage, plays the
action last played by another against it.

**TRANSPORT**: Any process
wherein movement of matter and/or energy from one part of a system to another
occurs. Energy moves with the matter being transported (latent heat transport
in movement of water vapor) or separately (electromagnetic radiation).
The movement of matter may be in bulk over global distances (ocean currents),
or over regional or local lengths (turbulent mixing of stack emissions;
diffusion of heat from surface).

**TRAVELING
SALESMAN PROBLEM:** A classic problem in operations research; the objective
is to find the shortest path a "traveling salesman" could follow in order
to visit each of a set of geographically distributed locations.

**TRIAGE:** A system for allocating
scarce resources; it provides the maximum resources to individuals of highest
priority, and few or no resources to individuals of lowest priority. Derived
from practices used to prevent medical systems from being overwhelmed when
there are many sick or injured.

**UNCORRELATED:** Independent.
See also **CORRELATION**.

**VARIABLE**: A condition that
can differ from one experiment to another. Variables may be ** CONTROLLED**
or

**VOTER MODEL:** A particular
form of interacting particle system in which every site (cell) is occupied
by a specific type of particle, the identity of each particle changing
with each iteration according to rules conditioned on the states of the
neighboring cells.

**ZONE OF TRANSITION**: