"X" is often used as the notation for a random variable or independent variable, as in the basic transfer function, Y=f(x). In that notation, y would be the dependent variable.

In practical use, X is the variable that you are experimenting on to see what impact it has on the output, Y. (Note that the cases must remain consistent within an equation, and x would be different than X. For this article, we are talking in general terms, and use the capital version of the letter to make them stand out in the writing.)

As an example, you may be looking at the impact temperature (X) has on curing time (Y) for a paint oven. The experiment may be looking for the 'sweet spot' in terms of cost to run the oven remaining productive.

"X" (Variable)