Can be an OS version, target name or SDK add-on, e.g. "2.1" or "android-7".
For example, to use the Google APIs add-on to get an Android 2.3 emulator with Google Maps support, enter "Google Inc.:Google APIs:9".

The available targets in your installation of the Android SDK can be listed with the command: android list target — the values shown within double quotes can be entered here.