Merriam-Webster Collegiate Dictionary - binary

 I. noun  (plural -ries)  Date: 15th century something made of or based on two things or parts: as  a. ~ star  b. a ~ number system  II. adjective  Etymology: Late Latin binarius, from Latin bini two by two — more at bin-  Date: 1597  1. compounded or consisting of or marked by two things or parts  2.  a. duple — used of measure or rhythm  b. having two musical subjects or two complementary sections  3.  a. relating to, being, or belonging to a system of numbers having 2 as its base the ~ digits 0 and 1  b. involving a choice or condition of two alternatives (as on-off or yes-no)  4.  a. composed of two chemical elements, an element and a radical that acts as an element, or two such radicals  b. utilizing two harmless ingredients that upon combining form a lethal substance (as a gas) ~ weapons  5. relating two logical or mathematical elements a ~ operation  6. of or relating to the use of stable oppositions (as good and evil) to analyze a subject or create a structural model the ~ opposition of male and female — Joan W. Scott