Definitions for "Lactam"
One of a series of anhydrides of an amido type, analogous to the lactones, as oxindol; a cyclic amide.
A lactam (the noun is a portmanteu of the wods lactone + amide) is a cyclic amide. Prefixes may indicate the ring size: β-lactam (4-membered), γ-lactam (5-membered), δ-lactam (6-membered ring). That order in the nomenclature is because beta β, gamma γ and delta δ are the second, third and fourth letters in the alphabetical order of the Greek alphabet, respectively.