1.NounA crystalline nonhygroscopic acid that is a semisynthetic form of penicillin, used in the form of its sodium or potassium salt C16H17KN2O5S, and has antibacterial action similar to benzylpenicillin and is more resistant to inactivation by gastric acids; 6-phenoxyacetamidopenicillanic acid, C17H20N2O6S.