The Collaborative International Dictionary of English v.0.48:

Twilight \Twi"light`\, n. [OE. twilight, AS. twi- (see Twice)
   + le['o]ht light; hence the sense of doubtful or half light;
   cf. LG. twelecht, G. zwielicht. See Light.]
   [1913 Webster]
   1. The light perceived before the rising, and after the
      setting, of the sun, or when the sun is less than 18[deg]
      below the horizon, occasioned by the illumination of the
      earth's atmosphere by the direct rays of the sun and their
      reflection on the earth.
      [1913 Webster]

   2. faint light; a dubious or uncertain medium through which
      anything is viewed.
      [1913 Webster]

            As when the sun . . . from behind the moon,
            In dim eclipse, disastrous twilight sheds. --Milton.
      [1913 Webster]

            The twilight of probability.          --Locke.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Twilight \Twi"light`\, a.
   1. Seen or done by twilight. --Milton.
      [1913 Webster]

   2. Imperfectly illuminated; shaded; obscure.
      [1913 Webster]

            O'er the twilight groves and dusky caves. --Pope.
      [1913 Webster]

WordNet (r) 3.0 (2006):

twilight
    adj 1: lighted by or as if by twilight; "The dusky night rides
           down the sky/And ushers in the morn"-Henry Fielding; "the
           twilight glow of the sky"; "a boat on a twilit river"
           [syn: dusky, twilight(a), twilit]
    n 1: the time of day immediately following sunset; "he loved the
         twilight"; "they finished before the fall of night" [syn:
         twilight, dusk, gloaming, gloam, nightfall,
         evenfall, fall, crepuscule, crepuscle]
    2: the diffused light from the sky when the sun is below the
       horizon but its rays are refracted by the atmosphere of the
       earth
    3: a condition of decline following successes; "in the twilight
       of the empire"