1. The degree of base-pairing (A opposite U or T, G opposite C) between two sequences of DNA and/or RNA molecules. 2. The degree of affinity, or fit, of antigen- and antibody-combining sites.
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com·ple·men·tar·i·ty .käm-plə-(.)men-'tar-ət-ē, -mən- n, pl -ties correspondence in reverse of part of one molecule to part of another: as
a) the arrangement of chemical groups and electric charges that enables a combining group of an antibody to combine with a specific determinant group of an antigen or hapten
b) the correspondence between strands or nucleotides of DNA or sometimes RNA that permits their precise pairing <evolution of the contemporary genetic code involving purine-pyrimidine \complementarity (Struther Arnott & P. J. Bond)>
Medical dictionary. 2011.