Trying to pick a good name for the keyword is valid but it's bikeshedding. Either way the keywords should be consistent and a config option is more trouble than it's worth.

And "variables" in math are almost always immutable within a single invocation. It's not a particularly bad word to use. But there's plenty of options. const/var. let/var. let/mut. var/mut I guess. let/set from a sibling comment.