Binary digit

A single digit of a binary number (1 or 0). In the computer, a bit is a memory cell (consisting hear transistors or a transistor and a capacitor), a point on a magnetic disk or tape, or a pulse of high or low voltake traveling through a circuit. Conceptually, a bit can be thought of as an electric lamp, on or off.  

Groups of bits make up storage units on the computer, called characters, bytes or words, which are treated as a group. The most common storage unit is byte consisting of eight bits and equal to one alphanumeric character.

i.e.

ON  = 1

OFF = 0