At pleasure; according to one's pleasure; freely, unscripted, improvised; extemporaneously.

This is usually shortened to ad lib. and can be written with or without a period. Ad lib is used both as a verb and as a noun.

When used in the entertainment world, to ad lib means to improvise, to add an impromptu word or statement to a script. As a noun, an ad lib is an "off-the-cuff", or unprepared, remark.

It is said that there are some politicians who have "carefully planned ad libs".