beta-lactam
noun
be·ta-lac·tam
variants:
or β-lactam \ ˌbāt-ə-ˈlak-ˌtam, chiefly British ˌbē-tə- 
\
\
Medical Definition of beta-lactam
: any of a large class of natural and semisynthetic antibiotics (as the penicillins and cephalosporins) with a lactam ring