Longman Dictionary of Contemporary English
The Collaborative International Dictionary
Metric \Met"ric\ (m[e^]t"r[i^]k), a. [L. metricus, Gr. ?: cf. F. m['e]trique. See Meter rhythm.]
Relating to measurement; involving, or proceeding by, measurement.

Of or pertaining to the meter as a standard of measurement; of or pertaining to the decimal system of measurement of which a meter is the unit; as, the metric system; a metric measurement.
Metric analysis (Chem.), analysis by volume; volumetric analysis.
Metric system, See metric system in the vocabulary.
Metric system \Met"ric sys"tem\n. A system of weights and measures originating in France, the use of which is required by law in many countries, and permitted in many others, including the United States; the system is also called by its French name, Le Syst[`e]me International de Unit['e]s (abbreviated SI). The principal unit of length is the meter (see Meter). From this are formed the are, the liter, the stere, the gram, etc. These units, and others derived from them, are divided decimally, and larger units are formed from multiples by 10, 100, 1,000, and 10,000. The successive multiplies are designated by the prefixes, deka (formerly deca), hecto, kilo, and myria (seldom used); successive parts by deci, centi, and milli. The prefixes mega and micro are used to denote a multiple by one million, and the millionth part, respectively; giga and nano denote multiples of one billion (1,000,000,000) and one billionth, respectively. The prefix for one trillion (1012) is tera, and for one trillionth (1012) is pico; for one quintillion (1015) peta, and for (1015) (one quintillionth) femto; for (1018) atto. See the words formed with these prefixes in the Vocabulary. For metric tables, see p. 1682.
Wiktionary
n. 1 The system of measurements developed in France in the 1790s and now used worldwide. 2 The modern version of that system, Systeme Internationale d'Unites (International System of Units), or SI system of measurements that is based on the base units of the meter/metre, the kilogram, the second, the ampere, the kelvin, the mole, and the candela. 3 Any variant of that system, that was not codified as SI, such as cgs
WordNet
n. a decimal system of weights and measures based on the meter and the kilogram and the second
Wikipedia
Condorcet, 1791). Four everyday measuring devices that have metric calibrations: a tape measure calibrated in centimetres, a thermometer calibrated in degrees Celsius, a kilogram weight, and an electrical multimeter that measures volts, amperes and ohms.
The metric system is an internationally agreed decimal system of measurement. It was originally based on the and the introduced by the First French Republic in 1799, but over the years the definitions of the metre and the kilogram have been refined, and the metric system has been extended to incorporate many more units. Although a number of variants of the metric system emerged in the late nineteenth and early twentieth centuries, the term is now often used as a synonym for "SI"The following abbreviations are taken from the French rather than the English
 SI: Le Système international d'unités
 CGPM: Conférence générale des poids et mesures
 CIPM: Comité international des poids et mesures
 BIPM: Bureau international des poids et mesures
 CIE: ''Commission Internationale de l'Eclairage '' or the " International System of Units"—the official system of measurement in almost every country in the world.
The metric system has been officially sanctioned for use in the United States since 1866, but the US remains the only industrialised country that has not adopted the metric system as its official system of measurement. Many sources also cite Liberia and Myanmar as the only other countries not to have done so. Although the United Kingdom uses the metric system for most administrative and trade purposes, Imperial units are widely used by the public and are permitted or obligatory for some purposes, such as road signs.
Although the originators intended to devise a system that was equally accessible to all, it proved necessary to use prototype units in the custody of national or local authorities as standards. Control of the prototype units of measure was maintained by the French government until 1875, when it was passed to an intergovernmental organisation, the General Conference on Weights and Measures (CGPM).
From its beginning, the main features of the metric system were the standard set of interrelated base units and a standard set of prefixes in powers of ten. These base units are used to derive larger and smaller units that could replace a huge number of other units of measure in existence. Although the system was first developed for commercial use, the development of coherent units of measure made it particularly suitable for science and engineering.
The uncoordinated use of the metric system by different scientific and engineering disciplines, particularly in the late 19th century, resulted in different choices of base units even though all were based on the same definitions of the metre and the kilogram. During the 20th century, efforts were made to rationalise these units, and in 1960, the CGPM published the International System of Units, which has since then been the internationally recognised standard metric system.
Usage examples of "metric system".
The metric system, you probably know, is the system by which the majority of the world measures things.
Clive is still using the metric system of measurement, with most measurements converted to English in the novel.
The metric system is superior to the hodgepodge used in Jupiter, but one remains most comfortable with what one is most familiar with.
I might have guessed its mass from the force it took to keep it there, but did any of us expect the aliens to calibrate their dials in the metric system?