In organic chemistry, the acyl group is a specific type of carbonyl group. A compound in the carbonyl group has three 'things' bonded to the central carbon atom, one of these things being a double-bonded oxygen C=O. This C=O double bond is what defines a carbonyl. Carbonyl groups always have two other things, though. If one of them is a CH3 group, then that whole unit, H3C-C=O, is an acyl group. It is also sometimes called an acetyl group.