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Ad libitum is Latin for "at one's pleasure" (at liberty); 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).
Music or rhythm 
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:
- to play the passage in free time rather than in strict or "metronomic" tempo (a practice known as rubato when not expressly indicated by the composer);
- to improvise a melodic line fitting the general structure prescribed by the passage's written notes or chords;
- to omit an instrument part, such as a nonessential accompaniment, for the duration of the passage; or
- in the phrase "repeat ad libitum," to play the passage an arbitrary number of times (cf. vamp).
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."
In biological field studies it can also mean that information or data were obtained spontaneously without a specific method.
Medical prescriptions may use the abbreviation ad lib. to indicate "freely" or that as much as one desires should be used.
Ad-lib is used to describe individual moments during live theatre when an actor speaks through their character using words not found in the play's text. When the entire performance is predicated on spontaneous creation, the process is called improvisational theatre.
In film the term ad-lib usually refers to the interpolation of unscripted material in an otherwise scripted performance. In interviews, Dustin Hoffman says he ad-libbed the now famous line, "I'm walking here! I'm walking here!" as "Ratso" Rizzo in Midnight Cowboy.
Live performers such as television talk-show hosts (e.g., Conan O'Brien, David Letterman) sometimes enhance their reputation for wit by the delivery of material that sounds ad-libbed but is actually scripted, and may employ ad-lib writers to prepare such material. Some actors are also known for their ability or tendency to ad-lib, such as Peter Falk (of the series Columbo), who would ad-lib such mannerisms as absent-mindedness while in character.
See also 
- Ehrlich, Eugene (1985). Amo, Amas, Amat and More. New York: Harper & Row, Publishers. p. 23.