A semi-automatic weapon moves each new bullet into place for you after you fire. You still press the trigger once for each shot, but the next shot can come very quickly, because you do not have to load that next bullet with your hands.

It is not fully automatic. A fully automatic gun keeps firing for as long as you hold the trigger down. People, and sometimes news reports, blur that line and call a semi-automatic gun “automatic.” Though actually, soldiers, police, and gun laws care about the difference: one trigger-press, one shot, versus a stream of shots.

The word is also a noun: a semi-automatic can mean that kind of gun. You will meet it in news, films, and games. In Taiwan this is almost always something you read about, not something in the house. People also call some cameras semi-automatic when the machine does part of the work — but this sense is the weapon.