It is in the Percy Jackson and the Olympians series. It is the food of the gods. (It's in other things too, but I would guess that you have heard of Percy Jackson.) It is in Greek mythology.
Ambrosia in Homer is the food of the Greek gods, but in Sappho or Alcman it is their drink. The word is the feminine form of the adjective ambrosios, which means not-for-mortals. It could also be...