What is a factor held constant?

In science, as in real life sometimes several 'factors' effect the outcome of an experiment. In order to make the problem easier to study one or more of these is 'held constant' or not allowed to change in order to see the effect of the other variables.

EX. Gas volume can be effected by both pressure and temperature. In order to understand the effect of pressure, Boyle kept the temperature constant. He then changed the pressure to see what happened to the volume of a gas. This gave him what is now called Boyle's Law: The volume of a gas varies inversely as the pressure when the temperature is held constant.