ETOPS is an acronym for Extended Operations. The term used to signify Extended Range Operation with Two-Engine Airplanes but the meaning was changed by the US FAA when regulations were broadened to include aircraft with more than two engines. It refers to the standards and recommended practices (SARPS) issued by ICAO for Part 121 aircraft to fly long-distance routes that had been off-limits to twin-engined aircraft, and subsequently to extended range operations of four-engined aircraft (such as the Boeing 747-8 Intercontinental).
There are different levels of ETOPS certification, each allowing aircraft to fly on routes that are a certain amount of single-engine flying time away from the nearest suitable airport. For example, if an aircraft is certified for 180 minutes, it is permitted to fly any route not more than 180 minutes single-engine flying time to the nearest suitable airport.
ETOPS applies to twins on routes with diversion time more than 60 minutes at one engine inoperative speed. For rules that also cover more than two engines, as in the case of the FAA, ETOPS applies on routes with diversion time more than 180 minutes for airplanes with more than two engines.