The Collaborative International Dictionary of English v.0.48:

Byzantine \By*zan"tine\ (b[i^]*z[a^]n"t[i^]n), a.
   Of or pertaining to Byzantium. -- n. A native or inhabitant
   of Byzantium, now Constantinople; sometimes, applied to an
   inhabitant of the modern city of Constantinople. [Written
   also Bizantine.]
   [1913 Webster]

   Byzantine church, the Eastern or Greek church, as
      distinguished from the Western or Roman or Latin church.
      See under Greek.

   Byzantine empire, the Eastern Roman or Greek empire from a.
      d. 364 or a. d. 395 to the capture of Constantinople by
      the Turks, a. d. 1453.

   Byzantine historians, historians and writers (Zonaras,
      Procopius, etc.) who lived in the Byzantine empire. --P.
      Cyc.

   Byzantine style (Arch.), a style of architecture developed
      in the Byzantine empire.

   Note: Its leading forms are the round arch, the dome, the
         pillar, the circle, and the cross. The capitals of the
         pillars are of endless variety, and full of invention.
         The mosque of St. Sophia, Constantinople, and the
         church of St. Mark, Venice, are prominent examples of
         Byzantine architecture.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Byzantine Empire
    n 1: a continuation of the Roman Empire in the Middle East after
         its division in 395 [syn: Byzantine Empire, Byzantium,
         Eastern Roman Empire]