A decimeter is a unit of measurement equal to one-tenth of a meter, or a little less than four inches. The prefix deci- means “tenth,” so a decimeter is literally a tenth of a meter.

While Americans rarely use decimeters in everyday life (they tend to use inches, feet, and yards), they're part of the metric system used in many countries. A decimeter is roughly the width of an adult's hand or the length of a large candy bar. Ten decimeters lined up equal exactly one meter.

Scientists and engineers sometimes use decimeters because they're a convenient middle ground: bigger than a centimeter (which is tiny) but smaller than a meter (which can be awkwardly large for measuring medium-sized objects). If you're measuring the width of a textbook or the height of a shoebox, decimeters would work well.

The abbreviation for decimeter is dm. So if you see “5 dm” in a math problem or science experiment, that means five decimeters, which equals 50 centimeters or a little less than 20 inches.