Astronomers use metal to mean elements other than hydrogen and helium.
Wow, Astronomers are lazy. It's not hard to make up new terms for things when the existing ones clearly don't fit. Heck, if making up a word was too difficult they could have used an arbitrary acronym.
Well, when most of what they have to work with is hydrogen, a whiff of helium, and a tiny smattering of literally everything else ever, it's kinda hard to blame 'em. ;p
If it's worth saying, but not worth its own post, even in Discussion, it goes here.