Miria, también myria, (cuyo símbolo es my) es un prefijo usado como los del Sistema Internacional de Unidades, si bien no pertenece a él, aunque lo hizo. Miria indica un factor de 104, o 10 000. Así, por ejemplo, un miriámetro son 10 kilómetros o 10 000 metros. Un miriagramo son 10 kilogramos o 10 000 gramos. Este prefijo proviene de la palabra griega μύριοι (innumerable), de la que también viene miríada, que significa también un gran número indeterminado de algo, significado que también adquiere este prefijo (así, miriápodo, de innumerables patas), acercándose más a su significado etimológico. Este prefijo fue adoptado en 1795, si bien no se mantuvo tras la estandarización y fijación de los prefijos en la Conferencia general de los pesos y las medidas de 1960.

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Myria- is a now obsolete decimal metric prefix equal to 104 (ten thousand). It originates from the Greek μύριοι (mýrioi) (myriad). The prefix was part of the original metric system adopted by France in 1795, but was not adopted when the SI prefixes were internationally adopted by the 11th CGPM conference in 1960. In 19th century English it was sometimes spelled myrio, in line with a puristic opinion by Thomas Young. The myriametre (10 km) is occasionally encountered in 19th-century train tariffs, or in some classifications of wavelengths as the adjective myriametric. The French mesures usuelles (1812-1839) did not include any units of length greater than the toise, but the myriametre remained in use throughout this period. In Sweden and Norway, the myriametre is still common in everyday use. In these countries this unit is called mil. Of units customarily used in trade in France, the myriagramme (10 kg) was the metric replacement for an avoirdupois unit, the quartier (25 pounds). Isaac Asimov's novel Foundation and Empire still mentioned the myriaton in 1952. The myria’s symbol of my ultimately led to its demise. In 1905 the Comité International des Poids et Mesures (CIPM) assigned it the symbol M, wishing to use only single-letter symbols. This meant that myriameter, for example, was abbreviated Mm. But in the first part of the twentieth century, electrical engineers began to use capital M for the prefix mega-, as in megawatt and megohm. This usage became so widely and firmly adopted that in 1935 the CIPM adopted the prefix “mega-” with “M” as its symbol, dropping the myria- entirely. In 1975, the United States, having previously authorized use of the myriameter and myriagram in 1866, declared the terms no longer acceptable.

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