The complete statement is: derivative of a matrix expression or derivative of an expression involving matrices. But this is usually clear from the context.
Yes; like I said, it's a reflex. I know that it's abuse of terminology and I shouldn't overthink it, but, there's always a bit of a nag in my head when I hear it.
I have a similar annoyance with the notation for probability. I think stuff like `p(a)` is a historical mistake. We would have been better served with a notation that would make clear the distinction between a domain as an entity in itself, the set of possible values in the domain, and the action/event of selecting one or more values from that domain.
But oh well. The naming problem is one of the hardest problems in science after all :)