This is the Sigil of the Baphomet, drawn by Anton Szandor LaVey, founder of the Church of Satan.
Before you Christians get all huffy, no, I am not Satanic; I don't even believe in Satan. This is the centerpiece for my left arm's sleeve. Check out my right arm's sleeve outline if you're interested because I explain the whole idea there.
The tattoo IS perfectly circular, but unfortunately when you tattoo a circle on an arm it looks distorted unless you hold your arm at the exact perfect angle.