ETOPS is the airline-industry abbreviation for Extended-range Twin-engine Operational Performance Standards.
ETOPS rules govern flights by twin-engine aircraft on routes where the aircraft may be far from a suitable airport after an engine failure or another major problem. Regulators approve aircraft and operators for specific diversion times, based on reliability, maintenance, crew procedures, and operational planning.
The system became increasingly important as reliable twin-engine airliners began replacing some four-engine aircraft on long-distance routes. Earlier regulations often required long routes to remain closer to diversion airports, which favored aircraft with more engines.
ETOPS is not a blanket permission to fly anywhere. Approval depends on the aircraft type, operator, route, weather, airport availability, and diversion time. The acronym is widely used even though modern regulations may apply broader extended-diversion concepts to aircraft with more than two engines as well.