The Boolean NOT operation is a negation of its input.

A Logic NOT gate symbol

That is, the output of a NOT gate is NOT its input.

Look at this Truth Table

In Out
0 1
1 0

In schematic logic diagrams this is represented as a small circle on an input, or output, or as a buffer (triangle) plus the small circle. It is also commonly represented in notation as an overscore on the negated terms.