An IUD is a small object a doctor puts inside the uterus — the place where a baby would grow — so that a pregnancy is much less likely. It may be plastic, copper, or a type that slowly gives out hormone. Once it is in, it can stay for years; you do not take it every day like a pill, and taking it out is also a clinic job. People choose it when they want long-term birth control they do not have to remember each morning. It does not protect against infections that pass through sex, only against pregnancy. The letters stand for intrauterine device: inside the uterus, a thing made for that job.

子宮環、避孕環（子宮內避孕器）。台灣常這樣說；中國大陸較常說節育環、宮內節育器。