binary (ˈbaɪnərɪ)  
—adj  
1.  composed of, relating to, or involving two; dual 
2.  maths, computing of, relating to, or expressed in binary notation or binary code 
3.  (of a compound or molecule) containing atoms of two different elements 
4.  metallurgy (of an alloy) consisting of two components or phases 
5.  (of an educational system) consisting of two parallel forms of education such as the grammar school and the secondary modern in Britain 
6.  maths, logic (of a relation, expression, or operation) applying to two elements of its domain; having two argument places; dyadic 
—n , ries  
7.  something composed of two parts or things 
8.  astronomy See binary star 
9.  short for binary weapon 
[C16: from Late Latin bīnārius; see 
binary number  
—n  
a number expressed in binary notation, as 1101.101 = 1 × 2³ + 1 × 2² + 0 × 2^{1} + 1 × 2^{0} + 1 × 2^{}^{1} + 0 × 2^{}² + 1 × 2^{}³ = 13 5⁄8 
Main Entry:  binary number 
Part of Speech:  n 
Definition:  either of the numbers 0 and 1, used in binary notation 
Example:  Binary numbers let you represent any amount just using the digits 0 and 1. 
Usage:  computing 
binary bi·na·ry (bī'nərē)
adj.
Characterized by or consisting of two parts or components; twofold.
Consisting of or containing only molecules having two kinds of atoms.
binary (bī'nərē) Pronunciation Key

Anything composed of two parts. In modern computers, information is stored in banks of components that act like switches. Since switches can be either on or off, they have a binary character, and we say that the computer uses “binary arithmetic” to do its work.