An inverter is an electronic component used in digital systems that takes the logical value of its input and complements it. So, if the input voltage is logically low, a high voltage appears on the output, and vice versa. The exact values of the high and low voltages depend on the semiconductor family. See CMOS, TTL.

Here are a few symbols for the same piece of hardware, depending upon the desired use. Also, a polarity indicator (wedge) might replace the bubble.

Hi -> Lo         Lo -> Hi           In | Out
 |\                |\                H |  L
-| \o-           -o| \--             L |  H
 | /               | /
 |/                |/