Renaming variables, functions and classes is indeed a kind of refactoring (one of the simpler types). The renaming was cataloged as a kind of refactoring in the classic refactoring text by Martin Fowler.
Definition of Refactoring:
Refactoring is the process of changing a software system so that it does not change the outer behavior of the code but improves its internal structure. [Martin Fowler]
Different types of transformation meet this definition: from small and insignificant, which could take 5 minutes, to major redesigns that take months.