How are performance classes for helicopters/rotorcraft defined?
Multi-engine helicopters have 3 Performance Classes defined at certification that is related to single engine performance and how this affects follow-on risks following an engine failure.  

Class 1 helicopters have massive reserve power and can continue flight on one engine.  They have engines that are way larger than necessary for normal operation and normally operate at a fraction of their power potential until called upon to do it all.  Risks following engine failure are low.

Class 2 helicopters have somewhat less reserve power, and can continue flight temporarily on one engine, but not indefinitely.  Risk factors are higher.

Class 3 helicopters have little reserve power and are generally forced down when one engine fails, although at a much gentler descent rate than an autorotation. Risk factors are highest. 

I was looking for a link that provides a good detailed description but haven't had much luck.