Vancian

Refeering to the author Jack Vance, a perpetrator of the 'dying earth' genre of fantasy fiction. In roleplaying games the term 'vancian' became notable because the Dungeons & Dragons magic system, specificaly the way Wizards use spellbooks and memorize a certain number of spells per day was based on the magic system Vance used in his books.

4E has dropped the spellbook memorization mechanic from previous editions but retained similarity to Vance's magic in the distinction between simpler at-will spells, one-time spells and complex rituals. 4E casters never run out of spells as older edition wizards did (due to memorization issues), but rely on at-will spells (can be cast continuously, without limit) and magic implements (ie wands) for a more Harry Potter feel.

The Vancian system obviously remains in its most pure form in the Jack Vance's Dying Earth RPG.