The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            I have, with much pains and assiduity, qualified
            myself for a nomenclator.             --Addison.
      [1913 Webster]

   2. Studied and persevering attention to a person; -- usually
      in the plural.
      [1913 Webster]

WordNet (r) 3.0 (2006):

assiduity
    n 1: great and constant diligence and attention [syn:
         assiduity, assiduousness, concentration]