Homunculus (Latin for "Little Human", plural Homunculi) refers to the mythological concept of an artificially created human, presumably brought into existence by way of alchemy. Though a common concept, well known and understood by alchemists in Amestris, the idea of homunculi is regarded as no more than a farce or faraway fantasy as no individual or group of alchemists has come anywhere close to successfully creating such a being in officially recorded history. Even so, alchemists have been forbidden by the Amestrian government to attempt or research in the transmutation of humans and State Alchemists have such a restriction as part of their creed.
However, off the official record, not only one, but several of these creatures have been created in arcane, sinister secrecy. In the most relevant sense of the word, "Homunculus" refers to seven individuals who make up the main antagonist group of the Fullmetal Alchemist series. These created humans, named for the seven deadly sins (Pride, Lust, Greed, Envy, Sloth, Gluttony and Wrath), are beings with incredible longevity, astonishing regenerative properties and a host of varied superhuman abilities. The Homunculi usually carry the mark of the Ouroboros upon their bodies and are identified as such by said mark.
Ouroboros refers to the ancient mystical and alchemical symbol representing a serpent or dragon devouring its own tail, so as to signify infinity and unity. Being a circle, it shows that infinity has no beginning and no end, while the dragon biting its tail shows that the beginning and end of infinity are the same place. All of the seven main Homunculi in the Fullmetal Alchemist series are presumed to have the mark located somewhere upon their person as if tattooed there. This particular Ouroboros displays a winged serpent with its tail in its mouth, creating a circle with its body. Encircled within is a hexagram, which is a symbol often associated with alchemy. One could perceive the Ouroboros as a sort of basis for the Transmutation Circles used in the series and it is possible that the Ouroboros may also symbolize a solar eclipse in some belief structures.
Before learning of their identities as homunculi, Edward referred to the individuals behind the odd circumstances in Amestris as "Members of the Ouroboros".