An antibiotic is a chemotherapeutic agent that inhibits or abolishes the growing of micro-organisms, such for the reason that bacteria, fungi, or protozoa.

The term originally referred to any agent with biological activity against active organisms; however, 'antibiotic' now is used to refer to substances with anti-bacterial, anti-fungal, or anti-parasitical activity.

The rudimentary widely used antibiotic compounds used in modern medicine were produced and isolated from support organisms, such as the penicillin class produced by fungi in the relative Penicillium, or streptomycin from bacteria of the genus Streptomyces.

With advances in organic chemistry many antibiotics are now also obtained by chemical synthesis, such as the sulfa drugs.

Many antibiotics are relatively minute molecules with a molecular weight less than 2000 Da.

Unlike previous treatments for infections, which often consisted of administering chemical compounds so as strychnine and arsenic, with high toxicity also against mammals, antibiotics from microbes had not one or few side effects and high effective target activity.

Most anti-bacterial antibiotics do not desire activity adverse to viruses, fungi, or other microbes.

Anti-bacterial antibiotics can be categorized based on their target specificity: 'narrow-spectrum' antibiotics target particular types of bacteria, such as Gram-negative or Gram-positive bacteria, while broad-spectrum antibiotics affect a wide range of bacteria.