two's complement

complement system, a cascade of proteins in the blood that form part of innate immunity

method of complements, a method to compute addition and subtraction in computer science

complementary DNA, DNA reverse transcribed from a mature mRNA template

complement good, a good often consumed together with another good in economics

complementation (genetics), a test to determine if independent recessive mutant phenotypes are caused by mutations in the same gene or in different genes

complementary color, in painting and optics

complement (complexity), relating to decision problems and complexity classes

complement (linguistics), a word or phrase having a particular syntactic role

complementary experiments, in physics

complementarity (molecular biology), a property whereby double stranded nucleic acids pair with each other

phonetic complement

complement (music), an interval that when added to another spans an octave

complement (mathematics) has many meanings - see this disambiguation page