Mainshock
The largest quake in a sequence; smaller ones before it are foreshocks, after it aftershocks.
The mainshock is the largest earthquake in a sequence. Quakes that come before it are called foreshocks, and those that follow are aftershocks. The labels are assigned by size, not timing, so a sequence can only be fully named in hindsight.
This creates a real difficulty: at the moment it happens, a foreshock looks identical to an ordinary quake or a mainshock. There is no reliable way to know in advance whether a given quake will turn out to be the biggest one or merely a warning of something larger. It is one more reason earthquake prediction stays out of reach.