Ad libitum is Latin for "at one's pleasure", often shortened to Ad lib.

Music

In music, this instruction appears in sheet music to indicate that a part can be left out, such as an unecessary accompaniment, or that a passage is to be played in free time rather than in strict tempo. This kind of freedom with the beat for expressive ends, when not explicitly indicated by the composer, is known in classical music as rubato. More generally, the phrase ad libitum can be used to indicate an improvisation.

Biology

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. For example, "The rat's ad libitum weight was about 320 grams." The term is used in laboratory studies which have shown that free-fed animals are often less healthy and have shorter lifespans than animals with restricted diets.