A girl who is hot/cute on an almost supernatural level, and so appealingly dressed as well, that she seems to be from a painting or drawing.
Her existence seems improbable yet somehow she is real, and so this burning beacon of raw sexuality falls between being 3D and 2D.
Did you see that girl who walked past just now!? She was practically 2.5D!
Graphical art style with 2D and 3D elements.
Troll: Sonic Rush and The New Super Mario Bros. are only 2D! they suck!
Ace Bandage Jr: No, they are 2.5D. The backgrounds are 3D