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]

Moby Thesaurus II by Grady Ward, 1.0:

88 Moby Thesaurus words for "assiduity":
   advertence, advertency, alertness, application, ardor,
   assiduousness, attention, attention span, attentiveness, awareness,
   bulldog tenacity, care, concentration, consciousness,
   consideration, constancy, diligence, dogged perseverance,
   doggedness, ear, earnestness, endurance, energeticalness, energy,
   engrossment, fervor, fidelity, heed, heedfulness, indefatigability,
   industriousness, industry, insistence, insistency, intentiveness,
   intentness, laboriousness, loyalty, mindfulness, note, notice,
   observance, observation, obstinacy, pains, painstaking,
   painstakingness, patience, patience of Job, permanence,
   perseverance, persistence, persistency, pertinaciousness,
   pertinacity, plodding, plugging, preoccupation, regard,
   regardfulness, relentlessness, remark, resolution, respect,
   sedulity, sedulousness, single-mindedness, singleness of purpose,
   slogging, stability, stamina, staying power, steadfastness,
   steadiness, stick-to-itiveness, strenuousness, stubbornness,
   tenaciousness, tenacity, thoroughgoingness, thoroughness, thought,
   tirelessness, unremittingness, unsparingness, unswerving attention,
   vehemence, zealousness