Musical Direction at (one's) pleasure; as one pleases: used to indicate that the marked section may be altered in tempo, ornamented, omitted, improvised, etc.
See ad libitum in American Heritage Dictionary 4
(ăd lĭbˈĭ-təm)
adjective Music
At the discretion of the performer. Used chiefly as a direction giving license to alter or omit a part.