x-height

In typography, the x-height or corpus size refers to the distance between the baseline and the mean line of lower-case 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 u, v, w, and z. (Curved letters such as a, c, e, m, n, o, r and s tend to exceed the x-height slightly, due to overshoot.) One of the most important dimensions of a font, x-height is used to define how high lower-case letters are compared to upper-case letters.