The metric system is an international system of measurement adopted almost universally, with the United States, Liberia and Myanmar being the only exceptions.
Considered nearly synonymous with the Standard International System of Units (SI), the metric system is sometimes called the meter-kilogram-second (MKS or mks) system, although that usage is mostly deprecated. The system is based on three fundamental units: the meter (m), which quantifies displacement, the kilogram (kg), which quantifies mass, and the second (s or sec), which quantifies time.
Even where the metric system has not been adopted, it is used almost exclusively within the sciences and is the internationally-accepted standard of measurement for science engineering, technology and mathematics (STEM).
The metric system was originally developed by scientists who were frustrated with the less math-friendly foot-pound-second (FPS) system. In theoretical and laboratory calculations, arithmetic involving FPS units is "messy." There are, for example, 12 inches in a foot, three feet in a yard, and 5280 feet in a statute mile; there are 16 ounces in a pound. Less common English units such as the rod, furlong, peck, and bushel seem, in modern scientific terms, to have been dreamed up ages ago without concern for common sense, although they are used by some agricultural and industrial people to this day.
The meter and kilogram are divided into fractional units, and enlarged into multiple units, according to power-of-10 prefix multiplier. For example, there are 100 centimeters (102 cm) or 1000 millimeters (103 mm) in a meter, 1000 meters (103 m) in a kilometer, 1000 grams (103 g) in a kilogram, and 1000 milligrams (103 mg) in a gram. This makes these metric units easy to work with in scientific notation. Time, however, is denoted in the same way as in the English system. There are 60 seconds in a minute, 60 minutes in an hour, and 24 hours in a mean solar day in both systems.
The Standard International System of Units provides formal definitions for the meter, the kilogram and the second and also specifies and defines four additional units: the Kelvin for temperature, the ampere for electric current, the candela for luminous intensity and the mole for material quantity.
See also: centimeter-gram-second (cgs) or small-unit metric system
Watch Matt Anticole's TED-Ed lesson, Why the metric system matters: