(1) In a game played with time limits, the time (if any) available after the main time period has been used up is referred to as overtime.
(2) A time system that involves overtime. Common types include Canadian overtime and Japanese byo-yomi. (The BGA uses "overtime" more narrowly, so that it excludes Japanese byo-yomi.) Some other types of overtime are described on the BGA website.
Robert Pauli proposed the following distinction between main time and overtime:
Players are in overtime if their time runs out and the time system in charge grants them a second life.
They are in their first overtime if this happended once, in their second if it happended twice, etc.