Assiduity \As`si*du"i*ty\, n.; pl. {Assiduities}. [L. assiduitas: cf. F. assiduite. See {Assiduous}.] 1. Constant or close application or attention, particularly to some business or enterprise; diligence. I have, with much pains and assiduity, qualified myself for a nomenclator. --Addison. 2. Studied and persevering attention to a person; -- usually in the plural.