Definitions of independent variable on the Web:

• (statistics) a variable whose values are independent of changes in the values of other variables
  wordnetweb.princeton.edu/perl/webwn

• The terms "dependent variable" and "independent variable" are used in similar but subtly different ways in mathematics and statistics as part of the standard terminology in those subjects. ...
  en.wikipedia.org/wiki/Independent_variable

• In an equation, any variable whose value is not dependent on any other in the equation; The variable that is changed in an experiment
  en.wiktionary.org/wiki/independent_variable

• A variable presumed to influence or precede another variable (dependent variable). The variable is systematically manipulated by the researcher to determine changes in the dependent variable. Also known as the experimental variable or moderating variable. ...
  www.prm.nau.edu/prm447/definitions.htm

• A variable that is controlled or manipulated by the researcher.
  www.marketresearchterms.com/i.php

• The factor/value altered by the experimenter during the scientific investigation. The changes made affect the value of the dependent variable. This quantity is plotted along the x-axis of a graph.
  mdk12.org/instruction/curriculum/science/glossary.shtml

• The variable for the input of a function.
  www.spsk12.net/departments/STAR/math/8/8M-vocabulary%20sort-0809.doc

• The variable that you change on purpose. (It is what you are testing the effect of.)
  www.mtsd.org/teachers/jobes/

• an input variable representing a precisely known condition of an experiment. Note that designation independent depends on the goal of any given ...
  gfit.sourceforge.net/vocabulary.html

• one which is not affected by another eg depth is not affected by water temperature but the water temperature will vary with depth (it is the dependent variable).
  www.tuition.com.hk/geography/i.htm

• Any of those variables of a problem, chosen according to convenience, that may arbitrarily be specified, and that then determine the other or dependent variables of the problem. The independent variables are often called the coordinates, particularly in problems involving motion in space. ...
  amsglossary.allenpress.com/glossary/browse

• A condition that is deliberately manipulated by scientists to test the response in an experiment. See also responding variable, operational definitions.
  www.environment.nelson.com/0176169040/glossary.html

• A treatment under experimental control. The researcher imposes several (at least two) different values of the independent variable on participants, and compares the results to determine whether the variable differentially affects behavior. ...
  instructional1.calstatela.edu/dweiss/Psy302/Glossary.htm

• in an experiment the factor that which is suppose to make "it" happen, the causative factor.
  psychexperiment.tripod.com/handoutonterms.htm

• A characteristic being measured or observed that is hypothesised to influence another event or manifestation (the dependent variable) within a defined area of relationships under study; that is, the independent variable is not influenced by the event or manifestation, but may cause it or ...
  www.bestlibrary.org/sc9/2007/03/vocabulary_proc.html