In the Dungeons & Dragons game, a template can be applied to a creature to enhance its abilities or to even create a new type of creature.
Templates can be either acquired or inherited. An acquired template can be applied to a creature at any point (assuming it meets the prerequisites for the template), while an inherited template is gained at birth (or creation). For example, undead are usually acquired templates, as they are usually created upon the death of a living creature. On the other hand, a half-dragon results from the mating of a dragon with another creature, and thus is inherited.
Templates were first introduced to the Dungeons & Dragons game in the Third Edition Monster Manual. Since then, they have appeared in every monster supplement, and often are included in the monster sections of other rulebooks.