Source : Webster's Revised Unabridged Dictionary (1913)

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).

   {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.

Source : WordNet®

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