XHEIGHT
X-height
In typography, the x-height or corpus size refers to the distance between the baseline and the mean line in a typeface. Typically, this is the height of the letter x in the font, as well as the u, v, w, and z. However, in modern typography, the x-height is simply a design characteristic of the font, and while an x is usually exactly one x-height in height, in some more decorative or script designs, this may not always be the case.The above text is a snippet from Wikipedia: X-height
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