name of the letter M, c. 1200, from Latin; the Greek name was mu. In printing, originally the square corresponding in dimensions to the capital M of that type.
Entries linking to em
M
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).
en n.
name of the letter "N;" in printing (1793), a space half as wide as an em.