The largest earthquake in a "cluster" of earthquakes. Mainshocks are sometimes precede by "foreshocks", and generally followed by aftershocks.
The largest in a series of earthquakes occurring closely in time and space. The mainshock may be preceded by foreshocks or followed by aftershocks. 
mainshock. The largest earthquake in a sequence, sometimes preceded by one or more foreshocks, and almost always followed by many aftershocks.