Noun
x-height (plural x-heights)
(typography) The distance between the baseline and midline in a typeface, typically equal to the height of the lower-case letter x.
Lowercase letter align with something that is known as the x-height, which is the height of the lowercase x. Source: Internet
When you’re pairing typefaces in a design, it’s a good idea to use two that are similar in x-height. Source: Internet