parameter

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A constant in an equation that varies in other equations of the same general form, especially such a constant in the equation of a curve or surface that can be varied to represent a family of curves or surfaces.
• noun One of a set of independent variables that express the coordinates of a point.
• noun One of a set of measurable factors, such as temperature and pressure, that define a system and determine its behavior and are varied in an experiment.
• noun Usage Problem A factor that restricts what is possible or what results.
• noun A factor that determines a range of variations; a boundary.
• noun Statistics A quantity, such as a mean, that is calculated from data and describes a population.
• noun Usage Problem A distinguishing characteristic or feature.

from The Century Dictionary.

• noun in mathematics,: The third proportional to any diameter of a conic section and its conjugate diameter: specifically this is the parameter of the former of these diameters. The parameter of the transverse axis is called the principal parameter, or the parameter of the curve.
• noun Any constant quantity entering into an equation
• noun A variable quantity of which the coordinates of a geometrical locus are direct functions. Thus, the coördinates of every universal algebraic curve can be expressed as rational functions of a single parameter.
• noun In crystallography, the ratio of the three axes which defines the position of any plane of a crystal; more specifically, the ratio belonging to the unit or fundamental plane for a given species: this axial ratio and the angular inclination of the axes constitute the crystalline elements for a species.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.
• noun (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.
• noun (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.
• noun (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.
• noun (Crystallog.) The ratio of the three crystallographic axes which determines the position of any plane; also, the fundamental axial ratio for a given species.
• noun The limits, guidelines, or assumptions from within which an activity is carried out.
• noun (Computers) A variable used in a calculation within a computer program which must be assigned a value before the calculation can be performed.
• noun A characteristic or element, especially one used as a criterion for evaluation or judgment.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun mathematics, physics A variable kept constant during an experiment, calculation or similar.
• noun programming A name in a function or subroutine definition that is replaced by, or bound to, the corresponding actual argument when the function or subroutine is called: a formal parameter.
• noun programming The value which is passed into the function to instantiate such a name; the argument or actual parameter.
• noun A characteristic or feature that distinguishes something from others.
• noun geometry 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.
• noun crystallography The ratio of the three crystallographic axes which determines the position of any plane.
• noun crystallography The fundamental axial ratio for a given species.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any factor that defines a system and determines (or limits) its performance
• noun a quantity (such as the mean or variance) that characterizes a statistical population and that can be estimated by calculations from sample data
• noun a constant in the equation of a curve that can be varied to yield a family of similar curves
• noun (computer science) a reference or value that is passed to a function, procedure, subroutine, command, or program