Bluetooth is a short-range wireless system: radio waves that let devices next to each other talk without a cable. You use it to join a phone to a speaker, a keyboard or printer to a computer, a watch to a phone, or earbuds to a laptop. It is for things in the same room, more or less — not for the whole internet. That longer, wider job is Wi‑Fi. The name is a brand and a standard, so it is usually written with a capital B. People also use it loosely as a verb in speech (“just Bluetooth the file”), which is informal. The English words are blue + tooth, but that is a translation of an old name, not a description of the technology. It copies Old Norse Blátǫnn (Danish Blåtand), the nickname of the 10th-century Danish king Harald Bluetooth. The companies that built the standard named it after him because he was remembered for joining separate peoples together — as the radio link joins separate gadgets. Nothing here is about blue teeth.