1. any substance that interferes with a chemical reaction, growth, or other biologic activity. 2. a chemical substance that inhibits or checks the action of a tissue organizer or the growth of microorganisms. 3. an effector that reduces the catalytic activity of an enzyme. ACE i's (angiotensin-converting enzyme i's) see angiotensin-converting enzyme inhibitors. angiogenesis inhibitor a group of drugs that prevent growth of new blood vessels into a solid tumor.