"An acetal is a molecule with two single-bonded oxygen atoms attached to the
same carbon atom."
"Traditional usages distinguish ketals from acetals (whereas a ketal has two carbon-bonded R groups and is formally derived from a ketone, an acetal has one or both carbon-bonded R groups as a hydrogen and is formally derived from an aldehyde). Current IUPAC terminology classifies ketals as a subset of acetals."
"Formation of an acetal occurs when the hydroxyl group of a hemiacetal becomes protonated and is lost as water. The carbocation ion that is produced is then rapidly attacked by a molecule of alcohol. Loss of the proton from the attached alcohol gives the acetal."