Definitions for "X-height"
A typographic term referring to the height of letters with neither ascenders nor descenders, such as the letter x. The x-height of a typeface plays a major role in its apparent size - typefaces with a high x-height look significantly larger than typefaces with a low x-height, even when both are set the same point size.
("X-height") Defined as the height of a lowercase letter, not including ascenders and descenders.
That part of a letter with no ascender or descender - an 'a' or an 'x' for example.