A micrometer ( /maɪˈkrɒmɨtər/ US dict: mī·krŏm′·ĭ·tər), sometimes known as a micrometer screw gauge, is a device incorporating a calibrated screw used widely for precise measurement of small distances in mechanical engineering and machining as well as most mechanical trades, along with other metrological instruments such as dial, vernier, and digital calipers. Micrometers are often, but not always, in the form of calipers.

Colloquially the word micrometer is often shortened to mike or mic (/ˈmaɪk/) (US dict: mīk′).

The word micrometer is a neoclassical coinage from Greek micros, "small", and metron, "measure". The Merriam-Webster Collegiate Dictionary says that English got it from French and that its first known appearance in English writing was in 1670. Neither the metre nor the micrometre nor the micrometer (device) as we know them today existed at that time. However, humans of that time did have much need for, and interest in, the ability to measure small things, and small differences; the word no doubt was coined in reference to this endeavor, even if it did not refer specifically to its present-day senses.