be·ta-lac·tam (b??t?-l?k?t?m, b??-)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.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.