noun, plural: avidities
The term may be used in chemistry to describe the dissociation-dependent strength of an acid or base. In proteins, avidity describes the binding intensity of multiple bond interactions between proteins.
In immunology, avidity refers to the strength of antibody-antigen binding. In this regard, avidity may be different from affinity. The latter being a term used to describe the strength of a particular bond, i.e. the affinity of an antibody for the antigen as the strength of a non-covalent association between an antigen-binding site with a single antigenic epitope1. For instance, IgM may have low affinity but it has high avidity due to its 10 weak binding sites contrary to the two strong binding sites of IgG.
Reference: 1Mak, Tak W. and Mary E. Saunders. Primer to the Immune Response. Amsterdam: Academic/Elsevier, 2008. 74.