The term arose in the late 19th century to describe the particular type of voice required to sing three Wagnerian roles: the title role in Der fliegende Holländer, Wotan/Der Wanderer in the Ring Cycle and Hans Sachs in Die Meistersinger von Nürnberg.
This type of Italianate baritone voice has a brighter tone colour and sings at a slightly higher tessitura than that possessed by the bass-baritone.
The term bass-baritone is roughly synonymous with the Italian vocal classification basso cantante; for example, in the Verdian repertoire, Philip II in Don Carlos is usually taken by a bass-baritone, while Ferrando in Il trovatore is sung by a true bass, though the two roles' ranges are very similar.
[3] Gilbert and Sullivan's Savoy operas usually featured a comic bass-baritone character, created to make use of D'Oyly Carte company member Richard Temple.
In short: the bass-baritone is a voice that has the resonant low notes of the typical bass allied with the ability to sing in a baritonal tessitura.