From Citizendium, the Citizens' Compendium
- : A family of means, including one's complement, two's complements, and variants, used in computer memory and mass storage to differentiate positive from negative numbers
- : A term used in Regular Language to describe sets of closed languages, which can be accepted by a finite automaton, neeting the criterion
- Complement (immunology) : A sequence of normally inactive proteins, which, when activated by foreign proteins of bacteria and other microorganisms, produce protein variants that variously open holes in the cell wall of invaders, and also opsonize the foreign cells to make them "tasty" to attack cells that will destroy them with phagocytosis
- Complement (linear algebra) : A pair of subspaces which form an (internal) direct sum.
- Complement (logic) : Add brief definition or description
- : A term for the crew of a ship, most often a military vessel but also used for civilian craft.
- : A concept, coined by Catherine Kousmine, for nutritional supplements for medical therapy.
- Complement (set theory) : The set containing those elements of a set (or "universal" set) which are not contained in a given set.