PROTON PUMP INHIBITOR - A group of anti-ulcer medications which work by binding to H+/K+ ATPase, an enzyme which is found on the secretory surface of parietal cells. It thereby inhibits the final transport of hydrogen ions (via exchange with K+) into the gastric lumen. Some examples of clinically used proton pump inhibitors include omeprazole (Prilosec) lansoprazole (Prevacid), and rabeprazole (Aciphex).