@Switch BlaydeIf you're interested in a more in-depth discussion of the history of typographic terminology, check out https://ilovetypography.com/2008/03/21/extreme-type-terminology/ (this is part 1 of a 5-part essay.
The third part of the link explains the em and en in far more detail:
The space between words is known as interword separation, interword spacing, word spacing or wordspacing, and can be described as loose, normal or tight. There are also specific blank spaces that relate to the size of the type. The em space, mutton or mutton quad is the width of a capital M, the en space, also known as half an em or a nut, is half that width.
In the days of metal type, the em space and en space were supplemented by even smaller spaces, such as the 3-em or 3-to-the-em space, a third of the width of an em space, the 4-em or midspace, one quarter of the width of an em space, and the 5-em space, or 5-to-the-em space, one fifth of the width of an em space. Nowadays, graphic designers tend to refer to the smaller spaces as, in order of their decreasing widths, a flush space, a thin space and the tiny hair space. Other spaces worth noting are the nonbreaking space, which refuses to be hyphenated, the figure space, the width of a monospaced number, and a punctuation space, the width of the simplest punctuation marks.