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, between antigen- and antibody-combining sites.
3. The degree of affinity, or fit, between an enzyme and a substrate.
The definition information for complementarity is provided by Stedman's. You can search our medical dictionary here.
Stedman's, part of Lippincott Williams & Wilkins, provides a comprehensive line of health-science publications for healthcare professionals and medical students.