The term "px," when uncapitalized, stands for "pixel." If "PX" is capitalized, it stands for "post exchange," a store on military bases where soldiers and other authorized personnel can purchase food, clothing and other goods.
In CSS Web markup language, it is often necessary to specify a certain pixel measurement to ensure Web pages display properly. In these cases, the measurements are denoted in "px," for example "margin: 1px."
On military bases, particularly U.S. Army bases, the post exchange is sometimes referred to as the "PX." However, this is an archaic terminology. More often, military members refer to the "BX," or base exchange; the "NEX," or Naval exchange; or the "AAFES," the Army and Air Force Exchange System.