penicillin

noun

pen·​i·​cil·​lin ˌpe-nə-ˈsi-lən 

1

: any of several relatively nontoxic antibiotic acids of the general formula C₉H₁₁N₂O₄SR that are produced by molds (genus Penicillium and especially P. notatum or P. chrysogenum) or synthetically and are used especially against gram-positive cocci

also : a mixture of such acids

2

: a salt or ester of a penicillin or a mixture of such salts or esters

Examples of penicillin in a Sentence

Recent Examples on the Web

Fungi have long been valuable to science, giving the world penicillin and other life-changing compounds. —Neetika Walter, Interesting Engineering, 1 Oct. 2025

Investigators misled the participants about the nature of the study and actively withheld treatment – including penicillin, which became the standard therapy in the late 1940s – in order to study the effects of untreated syphilis on the men’s bodies. —Stacie Kershner, The Conversation, 22 Sep. 2025

Nearly 100 years ago, in 1928, Scottish physician Alexander Fleming famously noticed that, in a moldy Petri dish, Penicillium notatum had destroyed the bacteria around it (even if penicillin would still be more than a decade away from development). —New Atlas, 20 Sep. 2025

Once overshadowed by penicillin, phage research is now getting a second look as scientists race to find new ways to fight infections. —Alice Gibbs, MSNBC Newsweek, 11 Sep. 2025

Word History

Etymology

New Latin Penicillium

First Known Use

1929, in the meaning defined at sense 1

Rhymes for penicillin

• ampicillin
• methicillin
• pyroxylin
• amoxicillin
• carbenicillin
• haematoxylin
• millon
• villain
• vanillin

Kids Definition

penicillin

noun

pen·​i·​cil·​lin ˌpen-ə-ˈsil-ən 

: any of several antibiotics or a mixture of these produced by penicillia or in the laboratory and used especially against round disease-producing bacteria

Etymology

from scientific Latin Penicillium "a genus of molds producing antibiotics," from Latin penicillus, literally, "little tail" — related to pencil

Medical Definition

penicillin

noun

pen·​i·​cil·​lin ˌpen-ə-ˈsil-ən 

1

: a mixture of relatively nontoxic antibiotic acids produced especially by molds of the genus Penicillium (as P. notatum or P. chrysogenum) and having a powerful bacteriostatic effect against various chiefly gram-positive bacteria (as staphylococci, gonococci, pneumococci, hemolytic streptococci, or some meningococci)

2

: any of numerous often hygroscopic and unstable amido acids (as penicillin G, penicillin O, and penicillin V) that have a structure in which a four-membered lactam ring shares a nitrogen and a carbon atom with a thiazolidine ring to which it is fused and that are components of the penicillin mixture or are produced biosynthetically by the use of different strains of molds or different media or are synthesized chemically

3

: a salt or ester of a penicillin acid or a mixture of such salts or esters