A NOT gate is a one-input, one-output logic circuit whose output is always the complement of the input. That is, a LOW input produces a HIGH output, and vice versa. When interpreted for a positive logic system, a logic ‘0’ at the input produces a logic ‘1’ at the output, and vice versa. It is also known as a ‘complementing circuit’ or an ‘inverting circuit’.
The NOT operation on a logic variable X is denoted as X or X- . That is, if X is the input to a NOT circuit, then its output Y is given by Y = X or X- and reads as Y equals NOT X. Thus, if X = 0 Y = 1 and if X = 1 Y = 0.