A bit is a binary digit, taking a value of either 0 or 1. Binary digits are a basic unit of information storage and communication in digital computing and digital information theory. Information theory also often uses the natural digit, called either a nit or a nat. Quantum computing also uses qubits, a single piece of information with a probability of being true.

The bit is also a unit of measurement, the information capacity of one binary digit. It has the symbol bit, or b.
Source: Wikipedia