be·ta-lac·ta·mase (b??t?-l?k?t?-m?s?, -m?z?, b??-)n. Any of various enzymes that hydrolyze and inactivate beta-lactam antibiotics such as penicillin, found in many antibiotic-resistant bacteria.[beta-lactam, cyclic amide occurring in penicillins (beta + lact(one) + am(ide)) + -ase.]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.