Coolness, to me, is how "cool" the item is, in the slangy sense of the word. It means "neat, amazing, impressive" - makes you say "wow." Let's see... definition C.2 on-site - fashionable and attractive. Well, sort of. It can be used as an interjection.
-How do you like my new latest-and-greatest-high-tech-toy? (the coolness fallacy: this thing is not really very useful.)
-Wow! Cool!