The Collaborative International Dictionary of English v.0.48:

Bosporus \Bos"po*rus\ (b[o^]s"p[-o]*r[u^]s), n. [L.]
   A strait or narrow sea between two seas, or a lake and a
   seas; as, the Bosporus (formerly the Thracian Bosporus) or
   Strait of Constantinople, between the Black Sea and Sea of
   Marmora; the Cimmerian Bosporus, between the Black Sea and
   Sea of Azof. [Written also Bosphorus.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

Bosporus
    n 1: a strait connecting the Mediterranean and the Black Sea;
         separates the European and Asian parts of Turkey; an
         important shipping route