In Googling around for a refresher, I found:
http://www.richstowell.com/howto.htm
...from which I quoteth:
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4. Definitions.
Aerobatics/Acrobatics.
FAR 91.303 defines aerobatic flight thusly: an intentional maneuver involving an abrupt change in an aircraft's attitude, an abnormal attitude, or abnormal acceleration, not necessary for normal flight. Note that aerobatic flight is not demarcated by a specific pitch attitude or bank angle. (Aerobatic flight is often mistakenly thought to occur only when an aircraft exceeds 30 degrees of pitch or 60 degrees of bank relative to the horizon. This 30/60 rule, which appears under FAR 91.307 (c), merely specifies the conditions under which parachutes must be worn by the occupants of an aircraft.) In the classical sense, the term aerobatics includes spinning, looping, and rolling an aircraft through 360 degrees of yaw, pitch, and roll.
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Now I have a problem with this subjective demarcation of "abrupt change" and "not necessary for normal flight" - after all a touch-and-go is "not necessary for normal flight", nor is a V1 cut or a power-on stall. We do plenty of manuvers that are part of the training curriculum, that might be regarded by an inexpert witness as stunts or abnormal.
You've got my attention!