Pepsin (from the Greek pepsis, meaning digestion) is a digestive protease released by the chief cells in the stomach that functions to degrade food proteins into peptides. Other important digestive proteases are the pancreatic enzymes trypsin and chymotrypsin.
Pepsin is expressed as a pro-form zymogen, pepsinogen, whose primary structure has an additional 44 amino acids. Hydrochloric acid released by the parietal cells converts pepsinogen to pepsin by cleaving off the additional amino acids outside the secreting cell to avoid digestion of the cells of the stomach and pancreas (like all cells, their membranes contain protein).
Pepsin was discovered by Theodor Schwann in 1836. It was the first animal enzyme to be discovered.
Pepsi and Pepcid AC are derived from the word pepsin. Pepcid AC reduces the acid and pepsin of basal, nocturnal, and stimulated gastric secretion. Pepsin is also used in the preparation of cheese.
Sources: DiaPharma, Wikipedia