Organic compounds containing a ring of atoms in which the group –CO.O– forms part of the ring. Lactones can be formed (or regarded as formed) by reaction of an –OH group in one part of a molecule with a –COOH group in the other to give a cyclic ester. This type of reaction occurs with γ-hydroxy carboxylic acids such as the compound CH2(OH)CH2CH2COOH (in which the hydroxyl group is on the third carbon from the carboxyl group). The resulting γ-lactone has a five-membered ring. Similarly, δ-lactones have six-membered rings. β-lactones, with a four-membered ring, are not produced directly from β-hydroxy acids, but can be synthesized by other means.
http://www.acdlabs.com/iupac/nomenclature/93/r93_521.htm Information about IUPAC nomenclature