myria-

An obsolete decimal multiplier prefix in the metric system, 1879-1935, indicating 10⁴, that is, 10,000, of the unit.

The myria’s symbol was one of the factors that led to its demise. In 1905 the CIPM assigned it the symbol M, so that myriameter, for example, was abbreviated Mm.¹ But in the first part of the twentieth century, electrical engineers began to use capital M for the prefix mega-, to mean one million, 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 a symbol for it, dropping the myria- entirely.² In addition, it had become clear that adopting prefixes for every power of ten was counterproductive. It is optimal to have names only for those exponents that are multiples of 3.

In 1982, the United States, having authorized use of the myriameter and myriagram in the Act of July 28, 1866, declared the terms no longer acceptable.³

1.
J. Ren� Beno�t.
Annex to the Proces Verbaux.
1905.

2.
Proces Verbaux volume 17 (1935).

Page 76.

3. Federal Register 8399-8400, February 26, 1982.