Definition - What does Circuit Tester mean?
A circuit tester is a device which is plugged into an electrical socket to make sure it is wired properly. A circuit tester has a series of lights that show the state of the wiring in the outlet, so the technician can verify that it has been installed properly.
A circuit tester is also known as a receptacle tester or outlet tester.
Techopedia explains Circuit Tester
A circuit tester plugs into a wall outlet to test how it is wired. The main reason is to verify that the outlet is actually delivering AC power to any device plugged into it. An AC outlet needs a live connection and a neutral connection at the very least, with most newer outlets also having a ground wire to prevent electric shock in case a device’s insulation fails. A multimeter could also be used for these checks, but a circuit tester is faster and more convenient for performing basic tests. Some circuit testers can check GFCI outlets.
However, circuit testers do have some limitations: they cannot detect a reversed neutral and ground wire or a bootleg ground where the neutral and ground pins have been wired together, whereas a multimeter can detect both.