be·ta-lac·tam 
(bā′tə-lăktəm, bē′-)
(bā′tə-lăktəm, bē′-)n.
1. A lactam composed of a four-membered ring, formed by intramolecular condensation of an amino acid in which the amino group is located at the beta position, or any compound containing this group.
2. A beta-lactam antibiotic.
[BETA, second (in the series of lactam rings, the first being the simplest) + LACTAM.]