Mm is an abbreviation for millimeter, a unit of measurement in the metric system. A millimeter is one-thousandth of a meter, which makes it quite small: about the thickness of a dime or the width of a grain of rice.

Scientists, engineers, and manufacturers use millimeters when they need precise measurements. A carpenter might measure a gap of 3 mm between two boards. A doctor might describe a small mole as 2 mm across. Camera lenses are often described by their millimeter measurements, like a 50 mm lens or a 200 mm telephoto lens.

You'll see mm written after numbers to show the unit: 15 mm, 250 mm. The metric system, which includes millimeters, is used in most countries around the world and in scientific work. Even in the United States, where people commonly use inches and feet, millimeters appear on rulers and are standard in fields like medicine and engineering.