pi·per·a·cil·lin (p?-p?r??-s?l??n)n. A semisynthetic broad-spectrum penicillin, C23H27N5O7S, used in its sodium form often in combination with the beta-lactamase inhibitor tazobactam, and active against a variety of gram-positive and gram-negative bacteria.[pipera(zine) + (peni)cillin.]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.