The NOT gate contain a single input and a single output and its symbol and truth table (truth table contains a table of all of the possible input values and their corresponding outputs values) is illustrated in Figure. It performs the operation of inversion, that means. The output of a NOT gate takes a 1 (high) state if the input takes the 0 (low) states and vice-versa. A circuit, which carry out a logic negation, is called a NOT circuit or inverter as it inverts the output with respect to the input.
Figure: (a) Symbol of NOT Gate, and (b) Truth Table