A cross-platform game programming library
Allegro 4 and Allegro 5 are cross-platform, libraries mainly aimed at video game and multimedia programming. They handle common, low-level tasks such as creating windows, accepting user input, loading data, drawing images, playing sounds, etc. and generally abstracting away the underlying platform. However, Allegro is not a game engine: you are free to design and structure your program as you like.
According to the Oxford Companion to Music, Allegro is the Italian for «quick, lively, bright». It is also a recursive acronym which stands for «Allegro Low LEvel Game ROutines». Allegro was started by Shawn Hargreaves in the mid-90's but has since received contributions from hundreds of people over the net.