I have always thought of rollback being an uncommanded N1 decrease with an ITT increase (sub-idle state). Flameout is N1 AND ITT decrease.

I have searched FAA documents for the "official" definition, but it is a term that appears to be used to describe a variety of situations where the engine "speed" is decreasing uncommanded.

http://www.airs-icing.org/AIRS_II/AI...06-206-739.pdf

The above is a great white paper on "rollback" due to ingestion of ice. When I am asked this question, I give the example of "rollback" due to ice accumulation/ingestion as described in this paper.