**Darwinism**
The theory of evolution as proposed by Charles Darwin, which
combined variation of inheritable traits with natural
selection. After the discovery of the physical mechanism of
genetics, this was further refined into neo-Darwinism.

**Delta Rule**
The perceptron learning rule that specifies that weight
changes should be proportional to the product of a weight's input and
the error (or delta) term for the perceptron.

**Derivative**
An expression that characterizes how the output of a function
changes as the input is varied. Unlike integrals, derivatives
can be calculated in an analytical manner very easily.

**Decision Problem**
A problem in which all questions take the form ``Is something a member
of a particular set?'' and all answers are either ``yes'' or
``no.''

**Detector**
A sensor that translates the state of a classifier's
environment into a message that is suitable for posting to
the message list of the classifier system.

**Determinant**
A quantity of a matrix that characterizes the amount of expansion
or contraction that the matrix inflicts on a vector when that
vector is multiplied by the matrix.

**Deterministic**
Occurring in a non-random manner such that the next state of a
system depends only on prior states of the system or the
environment. Perfect knowledge of previous states implies
perfect knowledge of the next state.

**Diagonal Matrix**
A matrix that has 0 entries along all nondiagonal entries,
i.e., only the main diagonal may have non-zero values.

**Difference Equation**
An equation that describes how something changes in discrete time
steps. Numerical solutions to integrals are usually
realized as difference equations.

**Differential Equation**
A description of how something continuously changes over time.
Some differential equations can have an analytical solution
such that all future states can be known without simulation of
the time evolution of the system. However, most can have a
numerical solution with only limited accuracy.

**Differentiation**
The act of calculating a derivative; the inverse operation of
calculating an integral.

**Diffusion Limited Aggregation**
A type of stochastic fractal formed by particles floating
about in a random manner until they stick to something solid.

**Discrete**
Taking only non-continuous values, e.g., Boolean or
natural numbers.

**Dissipative System**
A dynamical system that contains internal friction that deforms
the structure of its attractor, thus making motion such as
fixed points, limit cycles, quasiperiodicity, and
chaos possible. Dissipative systems often have internal
structure despite being far from equilibrium, like a whirlpool
that preserves its basic form despite being in the midst of constant
change.

**Diverge**
For algorithms or computers, to run forever and never halt; for
iterative systems (like the equations for the Mandelbrot
set), reaching a state such that all future states explode in size.

**Dot Product**
The inner product of two vectors.

**Dynamical System**
A system that changes over time according to a set of fixed rules
that determine how one state of the system moves to another state.

**Dynamics/Dynamical**
Pertaining to the change in behavior of a system over time.