(set theory, of set A "in" set B) The set containing exactly those elements belonging to B but not to A.

(grammar) A complement which is coupled to an object.

(grammar) A complement which is coupled to a subject.

(immunology) An aspect of the innate immune system that supplements the actions of the antibodies and phagocytic cells in clearing out pathogens from an organism

(mathematics) The relative pseudo-complement of a given element (of a Heyting algebra) with respect to the least element — the "zero" of that algebra.

(linear algebra, functional analysis) The set of all vectors which are orthogonal to a given set of vectors.

(biology) The air that can be drawn into the lungs in addition to the tidal air, by the deepest possible inhalation.

plural of complement system

(regular expressions) Characters which are excluded in a pattern.

(immunology) Any of a group of plasma proteins that bring about the destruction of pathogens through opsonization or assault on the cell membrane.