This is a historical definition; in modern use, it simply refers to the size of the font, and the word "em" no longer has any practical or significant meaning. In fact, the same Wikipedia article extends this evolution in its use and meaning in the next section :
One of them was traditionally defined as the capital width “M” in the current font and the size of the dot, since “M” usually used the full width of square “blocks” or “em-quads,” (also “mutton ATVs”), which are used in printing machines. However, in modern fonts, the character M is usually somewhat less than one. Moreover, as the term has expanded to include a wider variety of languages and character sets, its meaning has changed; this enabled the inclusion of these fonts, fonts, and character sets that do not include capital “M,” such as the Chinese and Arabic alphabets. Thus, em usually means the dot size of the font in question, which matches the height of the metal body on which the font was applied.
In particular, in terms of CSS, “em” does not necessarily refer to the capital width M for a particular font; it is just a relative value.
If you ask about the etymology of the word "em", Wikipedia itself contains only a link to Adobe Glossary , which has something to say more about this:
The general unit of measurement in a print shop. Em is traditionally defined as the width of the uppercase M at the current face and point size. It is more accurately defined as just the current size of the point. For example, in a 12-point type, em is a distance of 12 points.
It clearly does not mention, somewhere authoritative, that this is a phonetic representation of capital M, but, given its definition of the namesake, I would not rule out such a possibility.
BoltClock Apr 05 '13 at 6:15 2013-04-05 06:15
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