mutt,
Both!
If you want a reference there are a lot of textbooks around. The training manuals for any aircraft should cover it as well...
ICT_SLB,
The 'critical engine' is one that affects performance rather than systems capability. ETOPs aircraft have a number of unique terms used to desribe events and contingencies that differ slightly from normal OPs aircraft (e.g. 'adequate' airports), whereby the definiations may get crossed over a little.
Lancer