ETOPS - Extended Twin-engine Operations - allows a twin to be a certain distance from the nearest suitable airport. The distance is given in minutes of flying time on a single engine.
To get ETOPS rating, the engine type must have a certain demonstrated in-service reliability, which is measured in "mean time between failures", and the aircraft must have additional redundancies to assure all systems will function when an engine is out. This information is provided by engineering groups that specialize in reliability and system safety, and they are a prominent part of the engineering function at airframers.
Based on the engine reliability and level of redundancies, the aircraft is given an "xxx" minute ETOPS certification (typically 120 or 180 minutes), which means it must be within 120 or 180 minutes from the nearest airport during every part of its flight. This factor is used for planning long routes where intermediate airports are scarce, particularly (but not necessarily) over water.
Three and four (or more) engined aircraft do not require ETOPS certification, as their reliability is inherently adequate for dealing with engine-out situations. That's why over some South Atlantic and Pacific routes an ETOPS aircraft may require a longer route to comply with its rating than a 3 or 4-engine aircraft that can fly direct.
I hope this fills the void.
"In God we trust, everyone else bring data"