In typography, leading (/ˈlɛdɪŋ/ LED-ing) refers to the distance between adjacent lines of type; however, the exact definition has become confused. In the days of hand-typesetting, it referred to the thin strips of lead that were inserted into the forms to increase the vertical distance between lines of type; in this case, the leading would be defined as the difference between 2 quantities: the size of the type and the distance from one baseline to the next. For instance, given a type size of 10 points and a distance between baselines of 12 points, the leading would be 2 points; put another way, a leading of 2 points means there is a distance of 2 points from the bottom of the high line of type to the top of the low line of type. In modern times, though, there seems to be widespread use of "leading" to refer instead to just the distance from one baseline to the next, probably because of modern layout software tracks that quantity instead of a virtual strip of lead.
The term is still used in modern page-layout software such as QuarkXPress and Adobe InDesign. In consumer-oriented word-processing software, this concept is usually referred to as "line spacing" or "interline spacing", the latter of which is actually a more accurate description of the original meaning.