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):

indelible
    adj 1: cannot be removed or erased; "an indelible stain";
           "indelible memories" [syn: indelible, unerasable]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "indelible":
   absorbing, acute, colorfast, constant, deathless, deep, deep-dyed,
   deep-felt, deepgoing, double-dyed, dyed-in-the-wool, enduring,
   fadeless, fast, fixed, haunting, heartfelt, homefelt, immortal,
   imperishable, impregnable, incorruptible, indestructible,
   ineffaceable, ineradicable, inexpugnable, inextinguishable,
   ingrain, ingrained, invincible, invulnerable, keen, lasting,
   nagging, nonperishable, obsessive, penetrating, permanent,
   persistent, pervading, piercing, poignant, profound, quenchless,
   recurrent, sharp, undampable, undestroyable, undying, unerasable,
   unfading, unforgettable, unquenchable