Well. I think a gitar can be lots of things. You have Barritone guitars, tenor guitars. "regular" guitars and of course BASS guitars! It's just like with revolvers. They are a type of pistol, but hardly anyone refers to them as revolving pistols. Go figure.
The term bass is also not very precise if you want to get serious about it. You have the double (upright) bass, and of course there is also the bass as a deep male voice (singing). I would even be tempted to say that the term bass doesn't refer to an instrument at all, but the position in the musical notation system. The deepest/lowest sounding position that is.