Ad libitum is Latin for "at one's pleasure"; it is often shortened to "ad lib" (as an adjective or adverb) or "ad-lib" (as a verb or noun). The roughly synonymous phrase a bene placito ("at [one's] good pleasure") is less common but, in its Italian form a piacere, entered the musical lingua franca (see below).

As a direction in sheet music, ad libitum indicates that the performer or conductor has one of a variety of types of discretion with respect to a given passage:

Note that the direction a piacere (see above) has a more restricted meaning, generally referring to only the first two types of discretion. Baroque music, especially, has a written or implied ad libitum, with most composers intimating the freedom the performer and conductor have.

For post-Baroque classical music and jazz, see cadenza.

Ad libitum is also used in psychology and biology to refer to the "free-feeding" weight of an animal, as opposed, for example, to the weight after a restricted diet or pair feeding. For example, "The rat's ad libitum weight was about 320 grams." In nutritional studies, this phrase denotes providing an animal free access to feed or water thereby allowing the animal to self-regulate intake according to its biological needs. For example, "Rats were given ad libitum access to food and water."