Solo bass is really just what it says it is - bass on its own... it doesn't really matter what techniques you use, how fast you play or what you do. If all that's making noise is you and your bass, that's solo bass...
Some people refer to a bass player fronting the band as solo bass, ala John Patitucci or Stanley Clarke, but I think that's probably more accurately thought of as 'lead' bass...
cheers
Steve
www.steve-lawson.co.uk