Line-Spacing or Leading

In typography, leading (pronounced /ˈlɛdɪŋ/, rhymes with heading) refers to the amount of added vertical spacing between lines of type. In consumer-oriented word processing software, this concept is usually referred to as "line spacing". Leading may sometimes be confused with tracking, which refers to the horizontal spacing between letters or characters.
The word comes from lead strips that were put between set lines. When type was set by hand in printing presses, slugs or strips of lead (reglets) of appropriate thicknesses were inserted between lines of type to add vertical space, to fill available space on the page.
DocFamily supports line-spacing when formatting a document from XSL-FO to its intermediate format.