Complement

A complement is often something that completes something else, or at least adds to it in some useful way. Thus it may be:

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)
Complement (music), an interval that when added to another spans an octave
Aggregate complementation (music), the separation of pitch-class collections into complementary sets
Complementary color, in painting and optics
Complement good (economics), a good often consumed together with another good
Ship's complement, the number of persons in a ship's company, including both commissioned officers and crew

Mathematics and physics

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