The Collaborative International Dictionary of English v.0.48:

Indelible \In*del"i*ble\, a. [L. indelebilis; pref. in- not +
   delebilis capable of being destroyed: cf. F.
   ind['e]l['e]bile. See In- not, and Deleble.] [Formerly
   written also indeleble, which accords with the etymology of
   the word.]
   [1913 Webster]
   1. That can not be removed, washed away, blotted out, or
      effaced; incapable of being canceled, lost, or forgotten;
      as, indelible characters; an indelible stain; an indelible
      impression on the memory.
      [1913 Webster]

   2. That can not be annulled; indestructible. [R.]
      [1913 Webster]

            They are endued with indelible power from above.
                                                  --Sprat.
      [1913 Webster]

   Indelible colors, fast colors which do not fade or tarnish
      by exposure.

   Indelible ink, an ink not obliterated by washing; esp., a
      solution of silver nitrate.

   Syn: Fixed; fast; permanent; ineffaceable. --
        In*del"i*ble*ness, n. -- In*del"i*bly, adv.
        [1913 Webster]

              Indelibly stamped and impressed.    --J. Ellis.
        [1913 Webster]

WordNet (r) 3.0 (2006):

indelibly
    adv 1: in an indelible manner; "this tradition has left its mark
           indelibly upon the social, political, educational and
           industrial fabric of this country"