It's simple, but - I would argue - the wrong choice. A better choice (in my humble opinion) would be

const = constant
readonly = read-only

If it's not too late to change the keywords, that, to me, would be the way to go.

By which I mean, if it's at all feasable to consider ditching the keyword "invariant" altogether, and introducing a new keyword "readonly", then let's do it. "readonly" should indicate a read-only view of data which someone else might modify, and "const" should imply that no-one can modify it ever, no way, nohow.

I'm not quite sure why no one before now has suggested using "readonly" to mean read-only and "const" to mean constant, but seems kind of a no-brainer to me. You know - calling a thing what it is, instead of something it's not. I know I'd be dead confused if int meant float, for example.