What does "binary" mean?

Binary: 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 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.