Initials/Acronyms
Agaricus Bisporus
Because I don't know everything, I looked up 'acronym' in my dictionary before I started. It said 'a word made up from first letters', eg radar, laser. I suppose to be strictly accurate I should have said Initialism.
Incidentally, you guys replying only with initials are kinda proving my point.
I'd be with Kluge too if I knew what he meant