Legibility
describes how a typeface's glyphs can be correctly identified as characters and words, including how individual characters can be distinguished from one another. Legibility should not be confused with readability, although legibility does affect readability.
X-height
In typography, the x-height, or corpus size, is the distance between the baseline and the mean line of lowercase letters in a typeface. Typically, this is the height of the letter x in the font (the source of the term), as well as the letters v, w, and z.