**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]