"Collapsing wave function" is one of those phrases that physicists just toss around, along the same lines of, "computing the eigen values of the Hamiltonian" or "staying home alone on a Saturday night."
We're just so used to it that it might never occur to us that it needs additional explanation.*
*Not that "staying home alone on a Saturday night" really needs much additional explanation. Enjoy the rest of the book, nerd.