Avidity is a feeling of enthusiasm, a form of willingness and eagerness. If you’re a fan of girl detectives, you read all of the Nancy Drew books with avidity.
People who like something a lot are avid, like avid fans of Batman who see every movie and collect every comic book. The noun form is avidity, which refers to this quality of being enthusiastic and eager. The word avidity is from the Latin aviditatem for "eagerness.” When starting a new project, like writing a story, most people dive in with avidity. When you feel avidity, you're keen to do something — you really want to do it.