TBF server is what discord calls it. And a discord server can contain a multitude of channels with fairly extensive individual customisation.
So it goes quite a bit further than “a chat channel”.
As far as I know they still stick to "guild" as the official term, but virtually everybody calls a guild a "server"
They changed all the user facing docs to "server" a while back. Internally the code might still say "guild", but officially they've embraced calling them servers.