Also a title in the Belgariad, and Mallorean trilogies by David Eddings. The title is given to one of the disciples of the god Aldur who has attained mastery of sorcery by succesfully using the Will and the Word. The prefix "Bel" is added to the beginning of the sorcerer's original name, for example:
Garath => Belgarath
Garion => Belgarion
Din => Beldin
Makor = Belmakor

you get the idea.

The prefix "Pol" is used in much the same way for female disciples who have attained mastery of sorcery as well. But that is another story.