am·pi·cil·lin (?m?p?-s?l??n)n. A broad-spectrum semisynthetic penicillin, C16H19N3O4S, that is used, often in the form of its sodium salt, to treat infections of the intestinal, urinary, and respiratory tracts.[am(ino)- + p(en)icillin.]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.ampicillin (?æmp??s?l?n) n (Pharmacology) a semisynthetic penicillin used to treat various infectionsCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014am?pi?cil?lin (?æm p??s?l ?n) n. a broad-spectrum semisynthetic penicillin, C16H19N3O4S, effective against certain susceptible Gram-positive and Gram-negative bacteria. [1965?70; probably am(ino-benzyl)p(en)icillin, an alternate chemical name] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.