Technically no it's not a guitar. Bass traditionally is simply a large violin, and was later converted to be played like a guitar. This does not technically make it a guitar, so i guess you could say its still part of the orchestral string family. Of course there is the sinple fact that it might be considered a guitar simply because it has frets and upright basses do not. I, for one, do consider it to be a sort of bridge between the two worlds, considering the technicalities but also the fact that it shares some of the same aspects as a normal guitar.
In other words, yes I do generally accept [electric] bass as a guitar.
The Guitarists Guild of Gaia
The Original Guitar and Bass Guild
