Glossary 219
derivative the limit of a ratio formed by the difference in the
dependent variables to the difference in the independent variables as
the difference in the independent variables tends toward 0
differential equation an equation containing the derivatives of
an unknown function. The solution of a differential equation is the
function or functions whose derivatives satisfy the equation and any
subsidiary conditions
differentiation the act of computing a derivative
DNA (deoxyribonucleic acid) the molecule of heredity; the mecha-
nism that encodes all of the information required to enable a cell to
develop into an individual organism
ecliptic the circle formed by the intersection of the celestial sphere
and the plane containing the Earth’s orbital path
efficiency in thermodynamics the work performed by a cyclic heat
engine per cycle divided by the amount of heat absorbed per cycle
expressed as a percentage; the closer this quotient is to 100 percent,
the more efficient the engine
ellipse a closed curve obtained by the intersection of a right circu-
lar cone and a plane
ensemble In probabilistic modeling, the modeler assumes that the
phenomenon of interest can occur many times. The collection of all
such potential occurrences is the parent population. The ensemble
is a representative sample of the parent population. Study of the
ensemble reveals the average values, variances, and other statistical
features of the parent population. (Sometimes the word ensemble is
also used to refer to the parent population itself.)
entropy a measure of the amount of energy in a physical system that
is unavailable to do work
first law of thermodynamics the assertion that energy is a con-
served property
fluid dynamics the branch of physics that deals with the properties
of liquids and gases in motion