Cephalosporins: A group of ß lactam antibiotics that are similar to the penicillins in structure and activity as well as side effects. However, as a group the cephalosporins tend to have broader spectrum of activity and are more stable. In many cases they are more resistant to ß lactamse produced by some bacteria. The cephalosporins are classified according to a generation system (1 - 4), with increasing gram-negative activity for the latter generation drugs.