kil·o·me·ter (k

-l

m'

-t

r or k

l'

-m

't

r)

noun

A unit of length equal to 1,000 meters or 0.62 of a mile.

Note: The metric system is a system of measurement that is based on the number 10. Because 12 inches make a foot, and 3 feet make a yard, calculating the number of inches in a given number of yards or miles can often be cumbersome. In the metric system, multiplication is easy.

*Kilo-* is a prefix meaning "a thousand," so one kilometer is equal to a thousand meters, and one kilogram is equal to a thousand grams. Likewise, if you know something is 18 kilometers away, you can easily calculate that it's 18,000 meters away.

Some common prefixes in the metric system are:

• milli- "one thousandth"

• kilo- "one thousand"

• centi- "one hundredth"

• cento- "one hundred"

• deci- "one tenth"

• deca- "ten"

The basic units of measurement in the metric system are the gram, for weight; the liter, for volume; and the meter, for distance. The prefixes can be combined with these units to form different measurements: a centigram is a hundredth (1/100) of a gram; a milliliter is a thousandth (1/1000) of a liter.

But the metric system isn't limited to these units: a kilowatt is a thousand watts, and a millisecond is a thousandth of second.

There are even more prefixes for larger and smaller units!

• micro- "one millionth"

• mega- "one million"

• nano- "one billionth"

• giga- "one billion"

• pico- "one trillionth"

• tera- "one trillion"