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.
Antibiotics are medicines used to treat infections or diseases caused by bacteria. There are many different types of antibiotics. The best antibiotic for your infection will depend on what infection you have and the type of bacteria causing it.