Webster's English Dictionary

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penicillin

pen.i.cil.lin \.pen-*-'sil-*n\ n : any of several relatively nontoxic 
   antibiotic acids of the general const itution C9H11N2O4SR or a salt or 
   ester of one of these acids or a mixture produced by molds (genus 
   Penicillium and esp. P. notatum or P. chrysogenum) or synthetically and 
   used esp. against cocci