> A constant is a variable...
No, it is not. A constant is the direct opposite of a variable.
> An immutable variable..
There is no such thing. You can decide not to mutate it but variable is by definition mutable.
If you want to argue mutability, then you have to talk about the data structure or memory footprint of the constant or variable that it points to or represents, not the concept of variable or constant itself.
In other words, we can have var foo = 5 and const bar = 5. foo can be changed by being reassigned another value with simple foo = 6, whereas bar cannot as bar = 6 should cause panic/exception/... On the other hand, we can have var foo = {value: 5} and const bar = {value: 5} and now it depends on the language how it handles complex types like a struct/object as the operation now bypasses the guards on the variable/constant assignment itself. Will it guard against mutation or not? It should, but that is rarely the case. Hence, in most languages, we will be able to do foo.value = 6 and also bar.value = 6, even though we should not. But again, now we are arguing about the mutability of the data type or memory representation and not the variable/constant itself. Most languages don't care about mutability, so we have this flawed thinking where we are simply unable to strictly define what data is actually mutable and what data is not. Rust uses the borrow checker, that is one approach, but generally this should be properly handled by the language spec and compiler itself and we should not even have this conversation where programmers simply cannot make a distinction between variables and constants, let alone comprehend what those terms mean in the first place, as those meanings have been thrown out of the window by the folks designing the languages.