[garethrowlands]

The term 'variable' is from mathematics. As others have said, the values of variables do vary but they do not mutate.

[astrobe_]

Yes, and math has the notion of "free variable" and "bound variable" [1].

[1] https://en.wikipedia.org/wiki/Free_variables_and_bound_varia...

[1-more]

They are often called bindings, not variables, so as to make it clear that they are just a name for a thing that will not change.