Ouroboros

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. 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.

Before learning of their identities as Homunculi, Edward referred to the individuals behind the odd circumstances in Amestris as "Members of the Ouroboros".

Lust's Ouroboros is located on her upper sternum, just above her breasts.

Gluttony's Ouroboros is located on his tongue.

Envy's Ouroboros is located on his left thigh.

Greed's Ouroboros is located on the back of his left hand.

Wrath's Ouroboros is located on his left eye in the manga (the 2003 anime version of Wrath has his Ouroboros located on the sole of his right foot).

Sloth's Ouroboros is located on the back of his right shoulder in the manga (the 2003 anime version of Sloth has her Ouroboros located above her left breast).

Pride's Ouroboros has not been displayed in the manga, but the Pride that the 2003 anime refers to is the Homunculus known as Wrath in the manga.