Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 n. Mathematics 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.
 n. Mathematics One of a set of independent variables that express the coordinates of a point.
 n. 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.
 n. Usage Problem A factor that restricts what is possible or what results: "all the parameters of shelter—where people will live, what mode of housing they will choose, and how they will pay for it” ( New York).
 n. A factor that determines a range of variations; a boundary: an experimental school that keeps expanding the parameters of its curriculum.
 n. Statistics A quantity, such as a mean, that is calculated from data and describes a population.
 n. Usage Problem A distinguishing characteristic or feature.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. A variable kept constant during an experiment, calculation or similar.
 n. 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.
 n. The value which is passed into the function to instantiate such a name; the argument or actual parameter.
 n. A characteristic or feature that distinguishes something from others.
 n. 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.
 n. The ratio of the three crystallographic axes which determines the position of any plane.
 n. The fundamental axial ratio for a given species.
from the GNU version of the Collaborative International Dictionary of English
 n. 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.
 n.
 n. 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.
 n. (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.
 n. 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.
 n. The ratio of the three crystallographic axes which determines the position of any plane; also, the fundamental axial ratio for a given species.
 n. The limits, guidelines, or assumptions from within which an activity is carried out.
 n. A variable used in a calculation within a computer program which must be assigned a value before the calculation can be performed.
 n. A characteristic or element, especially one used as a criterion for evaluation or judgment.
from The Century Dictionary and Cyclopedia
 n. 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.
 n. Any constant quantity entering into an equation
 n. 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.
 n. 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 WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 n. any factor that defines a system and determines (or limits) its performance
 n. a quantity (such as the mean or variance) that characterizes a statistical population and that can be estimated by calculations from sample data
 n. a constant in the equation of a curve that can be varied to yield a family of similar curves
 n. (computer science) a reference or value that is passed to a function, procedure, subroutine, command, or program
Etymologies
Examples

ABCfunction () if [SomeConditionIsTrue] then echo "1" else echo "0" fi parameter = "Cool" flag = ` ABCfunction $parameter `

Pagkalos also provided more detail on the redirect that Mox implemented over the weekend, noting that the attack used an IFRAME injected into the title parameter of a personal group  another social networking feature of the Obama site  that then let Mox remotely call some malicious JavaScript.

If the value of documentTitle is empty, then an empty string is used for the title parameter on the URL.

'Here we extend the term parameter to cover global or @ 1986 ACM O89791197O/86/06000152 75C imported variables.

The title parameter can receive UTF8 text as well (e.g. clipboard's content retrieved with Transform):

A parameter is included to update the local cache.
Yweather Puts The Weather On Your OS X Desktop  Lifehacker Australia

The more interesting Kantian parameter is that youve got protuberant hippo eyes, Matthew.

The  ar parameter is the quality of the audio, and the extension of the filename on the last parameter tells ffmpeg what format to convert the file into.
The Complete Guide To Ripping And Converting Flash Videos  Lifehacker Australia

Moscow reported that the root cause of the observed strong structural oscillations was an error in parameter settings uploaded to the SM engine gimballing control system, which then caused a malfunction of a dynamic (frequency) control filter.

SM engine gimballing control system, error in parameter settings uploaded
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jfk commented on the word parameter
Boy, those French: they have a different word for everything!  Steve Martin
June 17, 2009
jfk commented on the word parameter
Electronic music used pure sounds, completely calibrated. You had to think digitally, as it were, in a way that allowed you to extend serial ideas into other parameters through technology.  Luc Ferrari (One Frenchman who didn't get the memo I guess)
June 16, 2009
adoarns commented on the word parameter
I had always taken the phrase "within those parameters" to set up a metaphor of like a simulation with certain presets, and the outcome matched to whatever was run within them.
February 18, 2008
inkhorn commented on the word parameter
While I agree, it seems that this battle has already been fought and lost. See #4 at
http://www.mw.com/dictionary/parameter.
December 13, 2006
fearraigh commented on the word parameter
The endemic misuse of this word as a synonym for 'perimeter'  i.e. "within those parameters"  is a constant source of irritation for me. The French have both 'périmètre' and 'paramètre' and manage not to confound them.
December 13, 2006