n. A person who tends to space out often. He or she does not respond when directly spoken to. The space cadet is not necessarily a person of low intelligence or a heavy drug user, but rather one who is so easily lost in reverie that he or she loses all awareness of the surrounding physical world.
Don't bother trying to get her attention, she's being a space cadet.
Person 1: Yo me and the boys are going out tonight, you wanna come?
Person 2: Can't, I'm busy playing 3D Pinball Space Cadet
Person 1: 3D Pinball Space Cadet? You must be super manly
When people are in their own little world, constantly "spaced out" of their current surroundings and positions. Often associated with not paying attention, or confusion towards otherwise simple tasks.
"Listen Slick, I'm not going to repeat the story a 3rd time just because you're being a Space Cadet."