The Collaborative International Dictionary of English v.0.48:

Diligent \Dil"i*gent\, a. [F. diligent, L. diligens, -entis, p.
   pr. of diligere, dilectum, to esteem highly, prefer; di- =
   dis- + legere to choose. See Legend.]
   1. Prosecuted with careful attention and effort; careful;
      painstaking; not careless or negligent.
      [1913 Webster]

            The judges shall make diligent inquisition. --Deut.
                                                  xix. 18.
      [1913 Webster]

   2. Interestedly and perseveringly attentive; steady and
      earnest in application to a subject or pursuit; assiduous;
      industrious.
      [1913 Webster]

            Seest thou a man diligent in his business? he shall
            stand before kings.                   --Prov. xxii.
                                                  29.
      [1913 Webster]

            Diligent cultivation of elegant literature.
                                                  --Prescott.

   Syn: Active; assiduous; sedulous; laborious; persevering;
        attentive; industrious.
        [1913 Webster]

WordNet (r) 3.0 (2006):

diligent
    adj 1: quietly and steadily persevering especially in detail or
           exactness; "a diligent (or patient) worker"; "with
           persevering (or patient) industry she revived the failing
           business" [syn: diligent, persevering]
    2: characterized by care and perseverance in carrying out tasks;
       "a diligent detective investigates all clues"; "a diligent
       search of the files" [ant: negligent]