Homunculus (Latin for "Little Human") refers to the mythological concept of an artificially created human, presumably brought into existence by certain means of alchemy. Though a common concept, well known and understood by alchemists in Amestris, the idea of homunculus 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 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 the eight individuals who make up the primary antagonistic force of the Fullmetal Alchemist series and one of the main antagonist forces in the crossover series. These created humans, named for the seven deadly sins of man (Pride, Lust, Greed, Envy, Sloth, Gluttony and Wrath), along with their Father, are beings with incredible longevity, astonishing regenerative properties and a host of varied superhuman abilities. The Homunculi created by Father (with the sole exception of Pride) usually carry the mark of the Ouroboros upon their bodies and are identified as such by said mark. Other common things that they share are a very dark hair color, and the same eye color.
The Homunculi are the eight primary antagonistic forces of the Fullmetal Alchemist series. The central antagonist is the original homunculus known simply as Father, who is a fully artificial human created through alchemy several hundred years before the start of the series in a country known as Xerxes. The other seven Homunculi are embodiments of his vices, and therefore extensions of himself. He did this in an attempt to become the ultimate being by purging himself of the seven vices which make humans imperfect.