(This is for the fanfiction "Crossover Together". Info may change from canon information)
Homunculus are a species of artificial being created through means of a Philosopher's Stone.
Homunculus hold resemblence to a human being in appearance, though one can distinguish a Homunculus from other human-like beings from the Oroboros symbol located somewhere on the body. The location of the symbol is various between individuals, as specific features differentiate one Homunculus from another.
Homunculus have a very high endurance range, and if a part of their body is horrifically damaged, a Homunculus is capable of healing itself, and even regrow missing limbs if need be. The length of time for a homunculus to heal however depends on the severity of the wound, and as long as their "core" is undamaged, they're unable to be killed.
The exact process in creating a Homunculus is unclear, but all Homunculi are made up of a core Philosopher's Stone: a type of mineral found notably in the Underworld. A Homunculus's entire being is connected to their stone, and if one were to remove it from the body, the Homunculus would cease to exist.
If one drains them of their magic energy first, however, the Homunculus will turn into a true human being, in the case of Envy.