What do we call an experience in which only one factor is allowed to vary?

A controlled experiment means that only one factor is changed at a time while the constant are kept.

Source: https://www.khanacademy.org/science/high-school-biology/hs-biology-foundations/hs-biology-and-the-scientific-method/a/experiments-and-observations#:~:text=A%20controlled%20experiment%20is%20a,all%20others%20are%20kept%20constant.