In the Toaru Majutsu no Index, Ambrosia appears as a small cone-like fruit that, when consumed, gives whoever has eaten it ageless immortality, making them forever appear in whatever form they have upon consuming it. The effects of the ambrosia not only gives the one who has eaten it leave from death of old age, but apparently any form of death as well, terrestrial or extra-terrestrial, as Ladylee Tangleroad personally experienced.
Moreover, anyone who has eaten the fruit can never use magic on their own, as it twists the cycle of their life force necessary for creating magic, as such one requires others to do magic for them.
It is unknown if the Imagine Breaker is able to negate the effects of ambrosia.
During the Crusades, Ladylee Tangleroad came upon an injured crusader, whom she ended up saving. As thanks for saving him the man gave ambrosia to Ladylee, who tried to turn down his offer at first, but eventually yielded. She later ate the fruit, and from then became immortal.