Succinctly put, the x-height is the height of the lowercase x in a given alphabet.
In the past x-height was stated by some as a proportion of the cap height. In digital fonts, the x-height is specified in the font as a proportion of the Em, typically with 1000 or 2048 units to the Em. Additionally, many typographers state the x-height as a proportion of the total vertical span of the lowercase, often limited to simply the ascender height. This practice is based on the realization that lowercase letters constitute approximately 95% of text.