Imperfect, or otherwise referred to as IMP, denotes an EX card (Rare EX, Ultra Rare EX, etc.) which will not reach perfect stats (being fully enhanced or not does not matter).
The "perfect" part refers to the two evolver cards having both been enhanced completely before evolving. Because they were not both enhanced fully before evolving, they would contribute 5% of their stats instead of 10%. Thus the new EX now has "imperfect" stats in that it will never be able to reach its maximum possible stats.