Certificate of Authenticity. It is a holographic sticker that comes with Microsoft software (including Windows and Office) and is usually attached to the side or bottom of a computer to indicate that the software installed on the computer is genuine (not pirated).
My copy of Windows XP is genuine; I have the COA stuck to the side panel of my computer.
I bought a copy of Windows for $10 from some guy in an alley, and it didn't come with a COA. No wonder Microsoft refuses to activate it for me!