APL, computer programming language based on (and named with the initials of) the book A Programming Language (1962) by Kenneth E. Iverson of IBM. It has been adapted for use in many different computers and fields because of its concise syntax. Statements are expressed with simple notations that have powerful built-in operational functions such as looping, sorting, and selection. Because of its origin in mathematical notation, APL had a unique set of characters. Once a popular language, it is not used often today for new programs.