To me it doesn't matter what the bass is played on, whether it be bass guitar, keyboard, double bass, tuba or even a stretched elastic band on a broom. As long as it makes a good groove & sounds good, that's what bass means to me.
I play several instruments but class bass as my main instrument because a good bassline can really make a song (not all songs need a bassline) & I used to do a lot of synth basslines (no longer have a synth). I'm also enjoying doing solo stuff on the bass too.