An antibiotic is a drug that kills or slows the growth of bacteria. Antibiotics are one class of antimicrobials, a larger group which also includes anti-viral, anti-fungal, and anti-parasitic drugs. They are relatively harmless to the host, and therefore can be used to treat infections. The term, coined by Selman Waksman, originally described only those formulations derived from living organisms, in contradistinction to "chemotherapeutic agents", which were purely synthetic. Nowadays the term "antibiotic" is also applied to synthetic antimicrobials, such as the sulfonamides. Antibiotic are small molecules with a molecular weight less than 2000. They are not enzymes.

Unlike previous treatments for infections, which included poisons such as strychnine and arsenic, antibiotic were labelled "magic bullets": drugs which targeted disease without harming the host. Antibiotic are not effective in viral, fungal and other nonbacterial infections, and individual antibiotic vary widely in their effectiveness on various types of bacteria. Antibiotic can be categorised based on their target specificity: 'narrow-spectrum' antibiotic target particular types of bacteria, such as Gram-negative or Gram-positive bacteria, whilst 'wide-spectrum' antibiotic affect a larger range of bacteria.

The effectiveness of individual antibiotic varies with the location of the infection, the ability of the antibiotic to reach the site of infection, and the ability of the bacteria to resist or inactivate the antibiotic. Some antibiotic actually kill the bacteria (bactericidal), whereas others merely prevent the bacteria from multiplying (bacteriostatic) so that the host's immune system can overcome them.

Oral antibiotic are the simplest approach when effective, with intravenous antibiotic reserved for more serious cases. Antibiotic may sometimes be administered topically, as with eyedrops or ointments.

Antibiotic can also be classified by the organisms against which they are effective, and by the type of infection in which they are useful, which depends on the sensitivities of the organisms that most commonly cause the infection and the concentration of antibiotic obtainable in the affected tissue.