• A malicious computer program which replicates itself until it takes over the entire system. This contrasts with a virus, which attaches itself to other programs, and which doesn't necessarily take over the entire system.


  • A single-celled micro-organism which replicates by growth and then division. Some are beneficial, some harmful. See examples under entry bacteria.


  • noun a microscopic organism. Some types are permanently present in the gut and can break down food tissue, but many can cause disease.

Origin & History of “bacterium”

Bacterium was coined in the 1840s from Greek baktḗrion, a diminutive of báktron ‘stick’, on the basis that the originally discovered bacteria were rod-shaped. At first it was sometimes anglicized to bactery, but the Latin form has prevailed. Related, but a later introduction, is bacillus (19th c.): this is a diminutive of Latin baculum ‘stick’, and the term was again inspired by the microorganism’s shape. Latin baculum is also responsible, via Italian bacchio and its diminutive form bacchetta, for the long French loaf, the baguette.