Definition of illegal. : not according to or authorized by law : unlawful, illicit; also : not sanctioned by official rules (as of a game)
antibiotic. n. 1. (Pharmacology) any of various chemical substances, such as penicillin, streptomycin, chloramphenicol, and tetracycline, produced by various microorganisms, esp fungi, or made synthetically and capable of destroying or inhibiting the growth of microorganisms, esp bacteria. adj.
Not to be confused with antibody. An antibiotic (or antibacterial) is something that kills bacteria or slows the growth of bacteria. They are used as medicines to cure diseases caused by bacteria. Antibiotics are no use against viruses. Penicillin, the first natural antibiotic, is produced by a fungus. Production started in 1939, and now it is made by chemical synthesis.