To ad-lib something, such as a performance or part of a performance, is to improvise it—that is, to make up words or music instead of saying, singing, or playing something that has been planned.

// The actor forgot his lines, so he ad-libbed.

See the entry >

“My real appreciation for [Céline] Dion grew in 2018, when I saw her perform in Vegas. I had agreed to attend with some friends, expecting a silly night of singing along to her hits like I was a kid again. It was the best live performance I had ever seen. Her singing was stunning, of course. She ad-libbed frequently, taking pleasure in showing off her range, and her voice was warm and supple.” — Caitlin Dickerson, The Atlantic, 18 July 2024

Let’s play a word game—just fill in a word of your choosing within the brackets in the following sentence, according to the category in italics: The word ad-lib comes from [language] and was first [past-tense verb] as a [part of speech] in the [ordinal number] century. If you jotted down “Latin,” “used,” “adverb,” and “eighteenth” you would be correct; ad-lib comes from Latin and was first used as an adverb in the eighteenth century. However, as the word game allows players to fill in whatever words they choose in accordance with their wishes, there were no wrong answers, a fact which also points toward the meaning of the verb ad-lib, which is a shortening of the Latin phrase ad libitum, meaning “in accordance with one’s wishes.” To ad-lib is to improvise, to go off-script, to say (or sing, or play on an instrument) whatever comes into your head in lieu of, or in spite of, a script or score. While ad-libbing may seem like a risky venture, some of the most famous lines in movie history were ad-libbed, from “Here’s looking at you, kid” to “You’re gonna need a bigger boat.” Isn’t that [adjective]?