The Collaborative International Dictionary of English v.0.48:

day-star \day"-star`\ (d[=a]"st[aum]r`), n.
   1. The morning star; the star which ushers in the day; --
      usually the planet Venus, when seen before and just after
      sunrise.
      [1913 Webster +PJC]

            A dark place, until the day dawn, and the day-star
            arise in your hearts.                 --2 Peter i.
                                                  19.
      [1913 Webster]

   2. The sun, as the orb of day. [Poetic]
      [1913 Webster]

            So sinks the day-star in the ocean bed,
            And yet anon repairs his drooping head,
            And tricks his beams, and with new-spangled ore
            Flames in the forehead of the morning sky. --Milton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

daystar
    n 1: a planet (usually Venus) seen just before sunrise in the
         eastern sky [syn: morning star, daystar, Phosphorus,
         Lucifer]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "daystar":
   Aldebaran, Canicula, Dog Star, Hesper, Hesperus, Lucifer,
   North Star, Phosphor, Phosphorus, Polaris, Sirius, Sol, Venus,
   Vesper, celestial body, chromosphere, comet, corona, evening star,
   fixed stars, heavenly body, living sapphires, lodestar,
   morning star, orb, orb of day, phoebus, photosphere, polar star,
   polestar, solar flare, solar prominence, solar wind, sphere,
   starry host, stars, sun



Easton's 1897 Bible Dictionary:

Daystar
   which precedes and accompanies the sun-rising. It is found only
   in 2 Pet. 1:19, where it denotes the manifestation of Christ to
   the soul, imparting spiritual light and comfort. He is the
   "bright and morning star" of Rev. 2:28; 22:16. (Comp. Num.
   24:17.)