I was afraid you would ask that (and hoped that someone else with more authority would answer). `immutable object` is used 5 times in [doc/manual/types.rst](https://github.com/JuliaLang/julia/blob/master/doc/manual/types.rst), so it is a safe bet for now.

I'm not sure I like the word "object" in this context, but that is a very different discussion.