Sorcha is probably more accurate in her definitions than I, as I'm really speaking relatively about the relationships of the three traditional members of the violin family.  Baritone violin tends to mean two quite different things nowadays; there's the instrument tuned an octave below normal violin that I mentioned:

Another Voice: The Baritone Violin

and a cello-tuned instrument with a larger body:

Baritone Violin

Further details of the latter, and of a whole set of "extra" violins, may be found here:

The Violin Octet

Malcolm