**Ecology**
The study of the relationships and interactions between organisms and
environments.

**Ecosystem**
A biological system consisting of many organisms from different
species.

**Edge of Chaos**
The hypothesis that many natural systems tend toward
dynamical behavior that borders static patterns and the
chaotic regime.

**Effector**
The part of a classifier system that can translate messages
into actions that can manipulate a system or an environment.

**Eigenvalue**
The change in length that occurs when the corresponding
eigenvector is multiplied by its matrix.

**Eigenvector**
A unit length vector that retains its direction when multiplied
to the matrix that it corresponds to. An (*n* * *n*) matrix
can have as many as *n* unique eigenvectors, each of which will have
its own eigenvalue.

**Embedding**
A method of taking a scalar time series and using delayed
snapshots of the values at fixed time intervals in the past so that
the dynamics of the underlying system can be observed as a
function of the previously observed states.

**Emergent**
Refers to a property of a collection of simple subunits that comes
about through the interactions of the subunits and is not a property
of any single subunit. For example, the organization of an ant colony
is said to ``emerge'' from the interactions of the lower-level
behaviors of the ants, and not from any single ant. Usually, the
emergent behavior is unanticipated and cannot be directly deduced from
the lower-level behaviors. Complex systems are usually
emergent.

**Entropy**
A measure of a system's degree of randomness or disorder.

**Environment**
If that which is under study is a system, then the rest of the
world is the environment.

**Equilibrium**
A state of a system that, if not subjected to perturbation,
will remain unchanged.

**Ergodic**
The property of a dynamical system such that all regions of a
state space are visited with similar frequency and that all
regions will be revisited (within a small proximity) if given enough
time.

**Euclidean**
Pertaining to standard geometry, i.e., points, lines, planes, volumes,
squares, cubes, triangles, etc.

**Euler's Method**
The simplest method of obtaining a numerical solution of a
differential equation. There are many other numerical techniques
that are more accurate; however, an analytical solution (i.e., a
closed form of an integral) is always preferred but not always
possible.

**Evolution**
A process operating on populations that involves variation among
individuals, traits being inheritable, and a level of
fitness for individuals that is a function of the possessed
traits. Over relatively long periods of time, the distribution of
inheritable traits will tend to reflect the fitness that the traits
convey to the individual; thus, evolution acts as a filter that
selects fitness-yielding traits over other traits.

**Evolutionary Stable Strategy (ESS)**
In game theory and biology, a strategy that, when possessed
by an entire population, results in an equilibrium such that
mutation of the strategy can never result in an improvement for an
individual. Always Defect is an ESS, while
Always Cooperate is not.

**Excitatory**
Refers to a neural synapse or weight that is positive such
that activity in the source neuron encourages activity in the
connected neuron; the opposite of inhibitory.

**Experimentation**
One process by which scientists attempt to understand nature. A
phenomenon is observed and/or manipulated so that changes in the
phenomenon's state can be seen. The resulting data can be used to
derive new models of a process or to confirm an existing model.
Experimentation is the complement of theorization. See also
simulation.

**Expert System**
A special program that resembles a collection of ``if ...
then'' rules. The rules usually represent knowledge contained by a
domain expert (such as a physician adept at diagnosis) and can be used
to simulate how a human expert would perform a task.