Source : Webster's Revised Unabridged Dictionary (1913)

Assiduous \As*sid"u*ous\, a. [L. assiduus, fr. assid?re to sit
   near or close; ad + sed[=e]re to sit. See {Sit}.]
   1. Constant in application or attention; devoted; attentive;
      unremitting.

            She grows more assiduous in her attendance.
                                                  --Addison.

   2. Performed with constant diligence or attention;
      unremitting; persistent; as, assiduous labor.

            To weary him with my assiduous cries. --Milton.

   Syn: Diligent; attentive; sedulous; unwearied; unintermitted;
        persevering; laborious; indefatigable.
        {As*sid"u*ous*ly}, adv. -- {As*sid"u*ous*ness}, n.

Source : WordNet®

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