its the variable your Testing....
"scientific fields such as biology, chemistry, and physics, the word variable is used to refer to a measurable factor, characteristic, or attribute of an individual or a system. In a scientific experiment, so called "independent variables" are factors that can be altered or chosen by the scientist. For example, temperature is a common environmental factor that can be controlled in laboratory experiments. "Dependent variables" or "response variables" are those that are measured and collected as data. An independent variable is presumed to affect a dependent one."
Your probably doing Dependant variables... http://en.wikipedia.org/wiki/Dependent_variable
Example.... You want to find out if different weights fall quicker....
You set up your tests (Dropping different weights from the same hights)
Here your independant variable is The weights and your results(which will/won't change is the time taken for the weights to drop) is the dependant variable
Whats the test about???