The Definition of a Control Variable in Sociology

A control variable is a variable that is held constant in a research analysis. The use of control variables is generally done to answer four basic kinds of questions:

Is an observed relationship between two variables just a statistical accident?
If one variable has a causal effect on another, is this effect a direct one or is it indirect with another variable intervening?
If several variables all have causal effects on the dependent variable, how does the strength of those effects vary?
Does a particular relationship between two variables look the same under various conditions?