The GNOME 2.2+ WindowManager. According to its author, Havoc Pennington: "Boring window manager for the adult in you. Many window managers are like Marshmallow Froot Loops; Metacity is like Cheerios." The first release of Metacity was version 2.3 - it has no need for your petty hangups about version numbers. It is configured entirely through GConf. The intent is to remove the traditional separation between WindowManager and DesktopEnvironment, presenting a single desktop interface.
The name does not mean meta-City as in an urban center, it means the state of being meta, just like opacity is the state of being opaque. It may also have something to do with the Meta key on Unix keyboards.