An antibacterial is a substance that kills bacteria or slows their growth.

It is sometimes used as a synonym for "antibiotic", but this term is more properly applied to the broader category of antimicrobial compounds.

In common usage, an antibiotic (from the Ancient Greek: ἀντί – anti, "against", and βίος – bios, "life") is a substance or compound that kills bacteria or inhibits their growth.

The term "antibiotic" was coined by Selman Waksman in 1942 to describe any substance produced by a microorganism that is antagonistic to the growth of other microorganisms in high dilution. This original definition excluded naturally occurring substances that kill bacteria but are not produced by microorganisms (such as gastric juice and hydrogen peroxide) and also excluded synthetic antibacterial compounds such as the sulfonamides. Many antibiotics are relatively small molecules with a molecular weight less than 2000 atomic mass units.^{[citation needed]}

With advances in medicinal chemistry, most antibiotics are now semisynthetic—modified chemically from original compounds found in nature, as is the case with beta-lactams (which include the penicillins, produced by fungi in the genus Penicillium, the cephalosporins, and the carbapenems). Some antibiotics are still produced and isolated from living organisms, such as the aminoglycosides, and others have been created through purely synthetic means: the sulfonamides, the quinolones, and the oxazolidinones. In addition to this origin-based classification into natural, semisynthetic, and synthetic, antibiotics may be divided into two broad groups according to their effect on microorganisms: Those that kill bacteria are bactericidal agents, whereas those that only impair bacterial growth are known as bacteriostatic agents.