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The Collaborative International Dictionary of English v.0.48 (gcide)

Parameter \Pa*ram"e*ter\, n. [Pref. para- -meter: cf. F.
param[`e]tre.]
1. A constant number which is part of a theory, function, or
calculation, whose value is not determined by the form of
the theory or equation itself, and may in some cases be
arbitrary assigned.
[PJC]

2. Specifically:
(a) (Math.) A term applied to some characteristic
magnitude whose value, invariable as long as one and
the same function, curve, surface, etc., is
considered, serves to distinguish that function,
curve, surface, etc., from others of the same kind or
family. --Brande & C.
(b) (Conic Sections) (in the ellipse and hyperbola), a
third proportional to any diameter and its conjugate,
or in the parabola, to any abscissa and the
corresponding ordinate.
[1913 Webster]

Note: The parameter of the principal axis of a conic section
is called the latus rectum.
[1913 Webster]

3. (Science) Any constant number which is required to
calculate values of observed phenomena according to a
theory, but the value of which must be determined by
experiment, and cannot be calculated from the fundamental
assumptions of the theory. In general, a theory which has
a large number of parameters, though it may accurately
predict experimental results, is considered as having less
explanatory power and as being less esthetically pleasing
than a theory with fewer parameters.
[PJC]

4. (Crystallog.) The ratio of the three crystallographic axes
which determines the position of any plane; also, the
fundamental axial ratio for a given species.
[1913 Webster]

5. The limits, guidelines, or assumptions from within which
an activity is carried out; as, new arrivals need to learn
the parameters of the research in our department.
[PJC]

6. (Computers) A variable used in a calculation within a
computer program which must be assigned a value before the
calculation can be performed; as, let's plug in the
parameters and see what the result is.
[PJC]

7. A characteristic or element, especially one used as a
criterion for evaluation or judgment; as, a useful
parameter for determining efficiency.
[PJC]