It comes from the Latin and means ‘at one’s pleasure’ and is commonly known as ‘Ad Lib’ or ‘Ad Libbing’.
In Music, Ad Libitum generally refers to an artist (singer or musician) who deliver spontaneously or improvise without restraint, prior practice or limit. However, playing ad libitum can mean a variety of things depending on its context:
• A performer can play a passage in free time rather than a specified tempo;
• A musician can improvise a melodic line of a passage as long as the melody fits within the existing harmonic structure of the passage;
• For a piece with more than one instrument, ad libitum means that one of the instruments is not an integral part of the harmony or melody and therefore, can be optional;
• A performer who ‘repeats ad libitum’ means he can play a passage as many times as he desires. If it is at the end of a song, he can repeat and fade-out.