1. [n]  a system of two stars that revolve around each other under their mutual gravitation
2. [adj]  consisting of two (units or components or elements or terms) or based on two; "a binary star is a system in which two stars revolve around each other"; "a binary compound"; "the binary number system has two as its base"
3. [adj]  of or pertaining to a number system have 2 as its base; "a binary digit"

1. \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.
   

2. \Bi"na*ry\, n.
   That which is constituted of two figures, things, or parts;
   two; duality. --Fotherby.
   

1. Base two. A number representation consisting of zeros and ones used by practically all computers because of its ease of implementation using digital electronics and Boolean algebra.

2. Any file format for digital data encoded as a sequence of bits but not consisting of a sequence of printable characters (text). The term is often used for executable machine code.

Of course all digital data, including characters, is actually binary data (unless it uses some (rare) system with more than two discrete levels) but the distinction between binary and text is well established.

3. A description of an operator which takes two arguments. See also unary, ternary.