1. when a female looses her breasts, usually associated with extreme dieting, exercise or a lack of nutrition;
2. a phenomenon caused by starvation, where boobs die/disappear, and often come back to haunt the individual when weight is gained back.
3. a growing epidemic among celebrities in which busty bossoms have been replaced with concave chests.
"Nicole Richie has ghost boobs! Where did they go?"
"Ghost boobs are so hot right now." - anonymous celebrity with ghost boobs