[ pen-uh-sil-in ]
/ ˌpɛn əˈsɪl ɪn /

noun Pharmacology.

any of several antibiotics of low toxicity, produced naturally by molds of the genus Penicillium and also semisynthetically, having a bactericidal action on many susceptible Gram-positive or Gram-negative cocci and bacilli, some also being effective against certain spirochetes.

Nearby words

  1. penhsi,
  2. penial,
  3. penicil,
  4. penicillamine,
  5. penicillate,
  6. penicillin g,
  7. penicillin v,
  8. penicillinase,
  9. penicillium,
  10. penicillus

Origin of penicillin

First recorded in 1925–30; penicill(ium) + -in2 Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for penicillin

British Dictionary definitions for penicillin


/ (ˌpɛnɪˈsɪlɪn) /


any of a group of antibiotics with powerful bactericidal action, used to treat many types of infections, including pneumonia, gonorrhoea, and infections caused by streptococci and staphylococci: originally obtained from the fungus Penicillium, esp P. notatum. Formula: R-C 9 H 11 N 2 O 4 S where R is one of several side chains

Word Origin for penicillin

C20: from penicillium

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for penicillin



1929, coined in English by Alexander Fleming (1881-1955), who first recognized its antibiotic properties, from Modern Latin Penicillium notatum (1867), the name of the mould from which it was first obtained, from Latin penicillus "paintbrush" (see pencil (n.)), in reference to the shape of the mould cells.

Online Etymology Dictionary, © 2010 Douglas Harper

Medicine definitions for penicillin


[ pĕn′ĭ-sĭlĭn ]


Any of a group of broad-spectrum antibiotic drugs obtained from penicillium molds or produced synthetically, most active against gram-positive bacteria and used in the treatment of various infections and diseases.

The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

Science definitions for penicillin


[ pĕn′ĭ-sĭlĭn ]

An antibiotic drug obtained from molds of the genus Penicillium and used to treat or prevent various infections caused by gram-positive bacteria such as streptococcus. Penicillin was the first of a class of antibiotics (whose names end in -icillin) that are derived from it and are active against a broader spectrum of bacteria. See Note at Alexander Fleming.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

Culture definitions for penicillin


An antibiotic that is used to treat infections caused by some kinds of bacteria. Penicillin, which is derived from a common kind of mold that grows on bread and fruit, was the first antibiotic discovered and put into widespread use.


Penicillin was first widely used during World War II.

The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.