The factor that changes in a controlled experiment is known as the variable, more specifically, it is the independent variable. In any controlled experiment, as per the scientific method, we only change one factor or variable at a time, called the independent variable and study its effect on a dependent variable. Thus, we change independent variable and study how it relates to a dependent variable. For example, we can study how the strength of tea depends upon the time a tea bag spends in the cup of hot water. Here, the time spent by the tea bag in the cup of hot water is an independent variable and tea strength is a dependent variable. We can test their inter relationship by carrying out a number of experiments, each with a different value of independent variable (say 10 sec, 30 sec, 2 min, etc.) and check the value of the dependent variable (maybe by using a professional tea tester).
Hope this helps.