The Collaborative International Dictionary of English v.0.48:

Superlative \Su`per*la"tive\, a. [L. superlativus, fr.
   superlatus excessive, used as p. p. of superiorferre, but
   from a different root: cf. F. superlatif. See Elate,
   Tolerate.]
   1. Lifted up to the highest degree; most eminent; surpassing
      all other; supreme; as, superlative wisdom or prudence; a
      woman of superlative beauty; the superlative glory of the
      divine character.
      [1913 Webster]

   2. (Gram.) Expressing the highest or lowest degree of the
      quality, manner, etc., denoted by an adjective or an
      adverb. The superlative degree is formed from the positive
      by the use of -est, most, or least; as, highest, most
      pleasant, least bright.
      [1913 Webster] -- Su`per*la"tive*ly, adv. --
      Su`per*la"tive*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

superlatively
    adv 1: to a superlative degree