Ambrosia

Ambrosia (アムブロシアー) is a small magic fruit that appears in the world of Toaru Majutsu no Index.

Etymology
The name comes from the of Greek mythology, where it is often times referred to as either the food or drink of the Greek gods, that confers ageless immortality upon whoever consumes it.

Principles
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 their youthful form upon consuming it. Anyone who eats the Ambrosia will  cease to age past their youth, will become entirely self-sustiained, they will heal/regenerate to perfect health and never become ill, and will never be able to die. Every injury shall cease; critical injuries will be healed in a matter of days. Complete destruction results in complete restoration. As such 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, which necessary for creating magic, making them free and independent of even the concept of reality, unbound to the subjects of life and death. 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, but it is unlikely, since Index, who knows about the Imagine Breaker, told  Ladylee Tangleroad there is no way for her to die.

Background
During the, 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.