The Collaborative International Dictionary of English v.0.48:

ampicillin \ampicillin\ n.
   1. a semisynthetic penicillin which can be considered as the
      common benzylpenicillin containing an amino group at the
      alpha benzyl position.

   Note: It is more active against certain gram-negative
         bacteria than is benzylpenicillin. It found wide use in
         treatment of bacterial infections, and has been sold
         under many trade names.

   Syn: D(-)-[alpha]-aminobenzylpenicillin.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

ampicillin
    n 1: semisynthetic penicillin (trade names Principen and
         Polycillin and SK-Ampicillin) [syn: ampicillin,
         Principen, Polycillin, SK-Ampicillin]