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".
Literally meaning is "at pleasure". This expression is popularly used as a noun phrase or modifier in English in the form "ad lib" to express absence of planning; for example, "His worst jokes were carefully planned ad libs."