One of many earthquakes that often occur during the days to months after some larger earthquake (mainshock) has occurred. Aftershocks occur in the same general region as the mainshock and are believed to be the result of minor readjustments of stress at places in the fault zone. (Noson, et.al., 1988)
An earthquake which follows a larger earthquake or main shock and originates in or near the rupture zone of the larger earthquake. Generally, major earthquakes are followed by a larger number of aftershocks, decreasing in frequency with time. (USGS National Earthquake Information Center, 1999)

This definition is in context to Earthquake. See more contextual defintions for Aftershock.