antibiotic: a chemical substance that kills or inhibits the growth of bacteria (a more precise term is antibacterial)
antimicrobial: a chemical substance that kills or inhibits the growth of microorganisms; this is a broader term than antibiotic, since it includes antibacterials (active against bacteria), antifungals (active against fungi; also known as antimycotics) and antiparasitic agents (active against parasites)
antiviral: a chemical substance that inhibits the replication of viruses and is used to treat viral infections. In contrast, viricides inactivate viruses, either within or outside the body