Mainshock
The earthquake of greatest magnitude in a series of earthquakes which occur over a relatively short period of time in a particular area.
Aftershocks
Additional earthquakes that occur after the mainshock and in the same geographic area. Usually an aftershock is smaller than the mainshock, but occasionally an aftershock may be strong enough to be felt widely throughout the area and may cause additional damage, particularly to structures already weakened in the mainshock.
Foreshock
An earthquake which occurs immediately preceeding a mainshock in the exact same area in which the mainshock occurs.