This is a measure of a character’s proficiency in a given profession, used in the game to separate combatants according to their combat potential. It is gained through experience. A spellcaster, assassin, fighter, monk, paladin, ranger or thief are considered elite combatants, able to cast spells, fight with a superior THAC0 and employ a wide range of sage fields, studies and abilities. Moreover, characters with a level have the benefit of better hit points. They will die if damage reduces them below -9 hit points.
For most classes, particularly spellcasters, learning the skills of any class requires years of education. It is therefore unusual that any non-levelled character will have the opportunity to matriculate to being levelled. However, non-levelled characters do collect experience, and in the same way as their more capable peers.