The Collaborative International Dictionary of English v.0.48:

Binary \Bi"na*ry\, a. [L. binarius, fr. bini two by two, two at
   a time, fr. root of bis twice; akin to E. two: cf. F.
   binaire.]
   Compounded or consisting of two things or parts;
   characterized by two (things).
   [1913 Webster]

   Binary arithmetic, that in which numbers are expressed
      according to the binary scale, or in which two figures
      only, 0 and 1, are used, in lieu of ten; the cipher
      multiplying everything by two, as in common arithmetic by
      ten. Thus, 1 is one; 10 is two; 11 is three; 100 is four,
      etc. --Davies & Peck.

   Binary compound (Chem.), a compound of two elements, or of
      an element and a compound performing the function of an
      element, or of two compounds performing the function of
      elements.

   Binary logarithms, a system of logarithms devised by Euler
      for facilitating musical calculations, in which 1 is the
      logarithm of 2, instead of 10, as in the common
      logarithms, and the modulus 1.442695 instead of .43429448.


   Binary measure (Mus.), measure divisible by two or four;
      common time.

   Binary nomenclature (Nat. Hist.), nomenclature in which the
      names designate both genus and species.

   Binary scale (Arith.), a uniform scale of notation whose
      ratio is two.

   Binary star (Astron.), a double star whose members have a
      revolution round their common center of gravity.

   Binary theory (Chem.), the theory that all chemical
      compounds consist of two constituents of opposite and
      unlike qualities.
      [1913 Webster]

WordNet (r) 3.0 (2006):

binary star
    n 1: a system of two stars that revolve around each other under
         their mutual gravitation [syn: binary star, binary,
         double star]