The term "ad-lib" can be defined as improvising or spontaneously creating something on the spot. As such, there are many synonyms in English that can be used to convey a similar meaning. Some of these synonyms include extemporaneous, impromptu, spontaneous, unscripted, unrehearsed, off-the-cuff, spur-of-the-moment, improvisational, and offhand. Each of these words captures the idea that something is being created without a lot of forethought or planning. Whether it's making up a speech on the spot or improvising a melody on a musical instrument, ad-libbing requires a certain level of creativity, spontaneity, and quick thinking.