MAMP stands for Mac OS + Apache + MySQL + PHP. It is the platform for a number of web-based applications, including Mediawiki. (When these applications are installed under Linux, the platform is called LAMP; when installed under Windows with IIS instead of Apache, the platform is called WIMP.)
MAMP is available in a prepackaged format that installs in a single application directory. Both the free package, MAMP, and the paid version, MAMP Pro, are available from here; there is also a SourceForge page.