LAMP is an acronym that stands for the set of open source software programs that are used as a platform for the development of web applications.
The LAMP stack consists of Linux, Apache, MySQL, Perl/Python/PHP
"LAMP represents the open source web platform. Most importantly, LAMP is the platform of choice for the development and deployment of high performance web applications. It is solid and reliable, and if Apache is any indicator, then LAMP sites predominate." (http://www.onlamp.com/pub/a/onlamp/2001/01/25/lamp.html)