This word stems from the French “avidité,” or directly from the Latin “avidus,” meaning “eager, greedy.” While the more common usage of “avidity” is as a synonym for “enthusiasm,” there’s a more technical usage as well. In biochemistry, “affinity” and “avidity” are closely related. “Affinity” is a word for how well a single antibody-antigen site binds, whereas “avidity” refers to the strength of all those interactions collectively. With avidity, binding strength depends on the effects that come from multiple proteins working together, because it’s easier for one to bind if another is already tethered nearby.