ad lib

Short definition from Wiktionary

Adverb

1. 1
   At pleasure.
2. 2
   At will.
3. 3
   As much as desired, to one's fill, without restriction.
4. 4
   Extemporaneously.

Adjective

1. 1
   Extemporaneous, impromptu.
2. 2
   Alternative form of ad lib.

Noun

1. 1
   An instance of something improvised, especially a line.
2. 2
   Improvised vocals added after the main vocals have been recorded which serve to emphasize the rhythm or fill the lyrical bar line.
3. 3
   Alternative form of ad lib.

Verb

1. 1
   To perform without a script.
2. 2
   To perform without preparation.
3. 3
   To perform extemporaneously.
4. 4
   To perform ad libs; to improvise lyrics after the main lyrics have been recorded.
5. 5
   To perform (lyrics) as ad libs.
6. 6
   Alternative form of ad lib.