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]