13th letter of the English alphabet, from Greek mu, from Semitic mem. It represents a very stable and unchanging sound in Indo-European, described by Johnson as "a kind of humming inward." The Roman symbol for 1,000; sometimes used in this sense in English 15c.-16c.; but in late 20c. newspaper headlines it stands for million. As a thickness of type, from 1680s (commonly spelled out, em).

Definitions of M
m (n.)
the basic unit of length adopted under the Systeme International d'Unites (approximately 1.094 yards);
Synonyms: meter / metre
m (n.)
concentration measured by the number of moles of solute per liter of solution;
Synonyms: molarity / molar concentration
m (n.)
the cardinal number that is the product of 10 and 100;
Synonyms: thousand / one thousand / " / k / chiliad / g / grand / thou / yard
m (n.)
a unit of information equal to 1000 kilobytes or 10^6 (1,000,000) bytes;
Synonyms: megabyte / mb
m (n.)
a unit of information equal to 1024 kibibytes or 2^20 (1,048,576) bytes;
Synonyms: megabyte / mebibyte / mb / MiB
m (adj.)
denoting a quantity consisting of 1,000 items or units;
Synonyms: thousand / one thousand / " / k