NOT Gate
The Boolean NOT operation is a negation of its input.
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.