Sign

"Though they are not warrior mages who employ powerful spells, witchers can cast simple magic spells that can prove effective when used properly. Witchers call these spells Signs and usually use them against monsters, though they also have non-combat applications." The witchers prefer single-handed weaponry because that leaves their other hand free to cast Signs.

The Witcher computer game
Five of the Signs come available over the course of the game: Aard, Igni, Yrden, Quen and Axii.

Leo, a witcher character unique to the game, tells in the Prologue that he cannot use Signs due to having injured his hand during training.