Extended-range bass
An extended-range bass is an electric bass guitar with a greater frequency range than a standard-tuned four-string bass guitar.
One way that a bass could be considered 'extended-range' is to utilize a tuning peg that allows for instant re-tuning, such as the popular 'Xtenders' made by Hipshot, however it is more so those bass guitars that have a low-B string such as the five string bass guitar and/or a high C-string - the six string bass guitar - that are most normally considered extended-range bass guitars. Basses have been made with seven, eight, nine, or even fifteen strings with extremely wide necks and custom pickups. These too are considered extended-range basses.
Michael Manring's 'Hyperbass' by Zon guitars and Les Claypool's main Carl Thompson piccolo bass are both four string basses but with necks that exceed the standard 24 frets (24 being the 'standard' for most commercially available bass guitars). Les Claypool's piccolo bass has 32 frets whereas Manring's Hyper Bass is a fretless instrument (however if it were a fretted bass it too would also exceed the 24th fret).