|Definition - A modifier that shouldn't be compared — i.e. be used in a comparative or superlative form — because it stands for a quality that is absolute and not a matter of degree.|
Example - Something is either unique or it is not. Because uniqueness is absolute, something can't be more unique than something else.
Note: Though prescriptivists claim that constructions such as more perfect are illogical, they are common in speech and writing and can convey useful information. For example, "Bob seemed perfect, but I think that John is definitely more perfect in all ways."