em quad

Etymology
Attested in 1841. From 1808, from 1683 , from m + , a metal type, which is as wide as the point size of the font. The origin of the letter m is unclear; perhaps because the type body of the letter M was approximately square during the early days of typesetting, and to contrast it with the n quadrat, the letter n being approximately half as wide.

Noun

 * 1)  A quad which has a fixed width of 1 em.

Usage notes
The em quad and the em space are usually synonymous as they both start as a space with a width of 1 em. However, in electronic publishing a few contrast the two by holding that if the font is condensed or expanded to alter the width of the characters, the em quad remains 1 em in width, while the em space is altered in width to the same proportion as the printing characters.