The representation of a number to base 2. It uses just the two binary digits 0 and 1, and this is the reason why it is important in computing. For example, 37 = 100101₂, since

Real numbers, not just integers, can also be written in binary notation, by using binary digits after a ‘binary’ point, just as familiar decimal representations of real numbers are written to base 10. For example, the number $\frac{1}{10}$, which equals 0.1 in decimal notation, has, in binary notation, a recurring representation: 0.00011001100110011…₂.