independent variable

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

• noun Mathematics A variable whose value determines the value of other variables.
• noun Statistics A manipulated variable in an experiment or study whose presence or degree determines the change in the dependent variable.

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

• noun algebra In an equation, any variable whose value is not dependent on any other in the equation.
• noun sciences The variable that is changed or manipulated in a series of experiments.

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

• noun (statistics) a variable whose values are independent of changes in the values of other variables