Complement

The word complement (not to be confused with a different word, compliment) has a number of uses:

complement (biology) is a group of proteins of the complement system, found in blood serum which act in concert with antibodies to achieve the destruction of non-self particles such as foreign blood cells or bacteria.
Complement (set theory). is a concept used in naive set theory. See
A complement (mathematics) is an operation that transforms an integer into its additive inverse.
In traditional music theory a complement (music) is the interval added to another, that is placed on top of another, so that their complete span is an octave.
In optics, complement refers to complementary colors.
In economics, a complement good is a good often consumed together with the good in question.

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