It's a higher level concept than the variable - a mutable binding is what people usually refer to as a variable, and an immutable binding is the correct term for what people refer to an "immutable variable" (an oxymoron, if you think about it).
It's not really an "immutable variable" - it's a local binding getting bound to different values on each scope entry.
EDIT: By the way, the `b` binding in your code can be modified. Did you mean `const b = a;` ?
It is, I just wanted to point out that the term "immutable variable" is sensible. I think a good way to put it is that b is a variable, and when the statement runs a value is bound to b. So the value bound to b varies yet b can be immutable, in contrast to a constant which is a binding to always the same value.
> Did you mean `const b = a;` ?
Fixed, thanks :)