Complement (music), an interval that, when added to another, spans an octave
Aggregate complementation, the separation of pitch-class collections into complementary sets
Complementary color, in the visual arts
Biology and medicine
Complement system (immunology), a cascade of proteins in the blood that form part of innate immunity
Complementary DNA, DNA reverse transcribed from a mature mRNA template
Complementarity (molecular biology), a property whereby double stranded nucleic acids pair with each other
Complementation (genetics), a test to determine if independent recessive mutant phenotypes are caused by mutations in the same gene or in different genes
Grammar and linguistics
Complement (linguistics), a word or phrase having a particular syntactic role
Subject complement, a word or phrase adding to a clause's subject after a linking verb
Phonetic complement
Complementary, a type of opposite in lexical semantics (sometimes called an antonym)
Mathematics
Algebra
Complement (group theory)
Complementary subspaces
Orthogonal complement
Schur complement
Algorithms
Complement (complexity), relating to decision problems and complexity classes
Complement operator (regular expressions)
Method of complements, in computer science
Radix complement
Diminished radix complement
Ones' complement
Two's complement
Discrete mathematics
Complement graph
Self-complementary graph, a graph which is isomorphic to its complement
Complemented lattice
Geometry
Complementary angles
Knot complement
Complement of a point, the dilation of a point in the centroid of a given triangle, with ratio −1/2
Logic
Complement (set theory)
Complementary event in probability
Logical complement
Bitwise complement
Complements in boolean algebra
Other uses
Complementary experiments, in physics
Complement good (economics), a good often consumed together with another good
Ship's complement, the number of persons in a ship's company