Slap bass is, in fact, the technique of playing the bass guitar with the thumb (as well as fingers) in a percussive manner. The thumb is "slapped" against the string, near to the fret/finger board to create a tone with a sharper attack (the very beginning of an acoustic waveform) than when a note is plucked.
This technique can also be augmented with "popping", which is to pull the string away from the board and release it so that it impacts the wood or metal to creates a very rhythmic sound. Slapping is usually done on the lower strings and popping the higher, often an octave above the root note, but this is by no means set in stone. Both techniques together are mostly used to create syncopated "funky" rhythms, and are referred to as "slap" or "slap and pop".
Proponents of this method include: