em space

Noun

 * 1)  A space which has a nominal 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.