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The Collaborative International Dictionary
Myria-

Myria- \Myr"i*a-\ [Gr. ? a myriad. See Myriad.] A prefix, esp. in the metric system, indicating ten thousand, ten thousand times; as, myriameter.

Wiktionary
myria-

pre. 1 innumerable: myriad, myriapod 2 (context obsolete English) In the metric systems of units, multiplying the unit to which it is attached by 10,000. Symbol: my

Wikipedia
Myria-

Myria- is a now obsolete decimal metric prefix equal to 10 (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.

Based on a proposal of Thomas Young, some early 19th century publications used myrio- as a synonym for myria-.

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.