overtime
Synonym discussion
Sense: noun/1
For noun/1, overtime usually means working hours beyond a person's normal schedule, especially in jobs with fixed shifts or office hours. Extra hours is the plainest near-synonym and works well in everyday speech. Additional hours is more formal and often appears in notices, contracts, or HR writing. Extended hours is wider: it may describe a store staying open longer, not only workers staying longer. Double shift is narrower and stronger, because it suggests working two full shifts back to back, not just one or two extra hours. Say 'I did overtime on Saturday' for ordinary extra work, but 'She worked a double shift at the hospital' for a much longer stretch.
Etymology
Overtime is a compound built from over and time. It first referred to time beyond a set limit, then became especially common for extra hours at work and later for the added period used in sports when a game is still tied.