Peptidases (old term: proteases, pronounced pro-tea-aces) are enzymes which break peptide bonds of proteins. They use a molecule of water to do so and are thus classified as hydrolases.
Peptidases occur naturally in living organisms, where they are used for molecular digestion and the breakdown of unwanted proteins. Peptidases can break either specific peptide bonds (limited proteolysis), depending on the amino acid sequence of a protein, or break down a complete peptide to amino acids (unlimited proteolysis).
Some viruseses, with HIV among them, depend on proteases in their reproductive cycle. Thus, protease inhibitors are developed as antiviral means.