that's a deceptively simple question.
in my experience, each manufacturer makes their own definition, and they are not always the same. you can get a hint for this by carefully reading datasheets and looking for test circuits and how they are tested.
in most cases however, you find that capacitors are given as peak DC. and the "rated value" is the value to which the manufacturer will guarantee the stated performance. then.. when you use it and it blows up and you question the manufacturer, they say "oh, there was a type-O in the datasheet" and then you read the findprint to see what the guarantee *really* is. hahaa... been there, done that!
Mr.Cool