See AdLib for the computer sound card manufacturer.

Ad lib (and ad-lib) are terms derived from the Latin ad libitum, meaning "at liberty". Ad lib is the adjective or adverb; ad-lib is the verb or noun form.

Most commonly, in drama, the quick-witted invention of dialogue to cover a performer's memory lapse would be an example of an ad-lib. Or, a director might encourage performers to ad-lib in a particular show. (The term ad-lib usually refers to the interpolation of unscripted material in an otherwise scripted performance. When the entire performance is grounded in spontaneous creation, the process is usually called improvisation).

Live performers such as television talk-show hosts 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.

In music, the term ad lib is used in a similar way to mean an improvised passage. It is also an instruction found in sheet music; see ad libitum.

Medical prescriptions may use the abbreviation ad lib. to indicate "freely" or that as much as one desires should be used.