In an experiment following the scientific method, a constant is a variable that cannot be changed or is purposely not changed during the experiment. Some constants are purposeful and selected by the scientist to control an experiment while others are more universal and beyond a researcher's control.
Any experiment conducted according to the scientific method will have constant variables and experimental variables. Experimental variables are those variables a scientist chooses to change so she may study the effects of the change. Constants are variables that are kept the same to ensure all effects being studied and measured are the result of an experimental variable.
If a researcher wanted to study the effects of temperature on the growth and development of garden snakes, the experimental variable for the experiment would be temperature. All other variables would need to remain consistent to avoid invalid data. The size of the cage, amount of light, food and many other variables would need to remain constant to ensure accurate results and a valid study. Those variables are constants. Some variables are not under a scientist's control, but are still considered to be constants. These constants are called universal constants and include gravity, the speed of light and electronic charge. Universal constants do affect experiments but will be constant through an experiment without being controlled by the scientist.