One of a class of amides

Wiktionary Word definitions in Wiktionary
n. (context organic chemistry English) Any of a class of cyclic amides, that are the nitrogen analogs of lactones, formed by heating amino acids; the tautomeric enol forms are known as lactims.

Wikipedia Word definitions in Wikipedia
A lactam is a cyclic amide . The term is a portmanteau of the words lactone + amide .

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Lactam \Lac"tam\, n. [Lactone + amido.] (Chem.) One of a series of anhydrides of an amido type, analogous to the lactones, as oxindol; a cyclic amide. beta-lactam or [beta]-lactam , a lactam in which the amide bond is contained within a four-membered ring, ...