1. <mathematics> 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. <file format> 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. <programming> A description of an operator which takes two arguments. See also unary, ternary.