independent variable :

a variable that is hypothesised to predict a dependent variable.

An independent variable in a research project is a variable that is believed- or hypothesized, to predict a dependent variable.

For instance, some epidemiological research focuses on the predictors of heart disease. Researchers collect data from many individuals, sometimes thousands of individuals, on their history of heart disease, and many other factors. One such study looked at the following for each individual: age, gender, weight, height, drinking habits, blood pressure, cholesterol levels, mother's health history, father's health history, daily exercise, and number of visits to the doctor per year. In this case, "incidence of heart disease" is the dependent variable. All the other variables, such as age, gender, weight, drinking habits, and so on, are all independent variables. They may or may not predict the dependent variable.

The term "independent variable" applies strictly to the context of one single research endeavor. In another study, the independent variable might be a dependent variable. In the above example, some researchers are interested in what causes people to gain weight. In their research, weight is a dependent variable, not an independent variable.