Myria-

Myria- (symbol my) is a now obsolete decimal metric prefix denoting a factor of 104 (ten thousand).

[8] Also, in 19th century English it was sometimes spelled myrio,[9][10] in line with a puristic opinion by Thomas Young.

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.

[13] 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.

[14] In 1975, the United States, having previously authorized use of the myriameter and myriagram in 1866, declared the terms no longer acceptable.