There are three main types of variables in scientific investigations:
1) Independent variables - variables that change (intentionally) in order to see their effect on another variable.
2) Dependent variable - is measured quantitatively or qualitatively and is affected by the independent variables.
3) In order for the results to be valid, the dependent variable can only be affected by the independent variable, so somethings need to be kept CONSTANT. The things that need to be kept constant are called CONTROLLED VARIABLES. Even if controlled variables are affecting the dependent variable(s), which they usually do, keeping them constant will ensure that all trials of an investigation were equally impacted by the controlled variables, therefore allowing one to see the impact of an independent variable on the dependent variable.
All in all, a 'constant' in a science experiment should actually be called a 'controlled variable' and the description of such a variable is described in the third point. Hope this clarifies.