In typography, the vertical space between lines of text is called leading.
The term comes from traditional metal typesetting, in which printers placed thin strips of lead between lines of type to increase the distance from one baseline to the next. It is pronounced “ledding,” referring to the metal, rather than “leeding.”
Leading is often described through a type-size and baseline measurement such as “10 on 12 point.” In that example, the type is 10 points high and the baselines are 12 points apart, leaving 2 points of leading. Modern layout programs still use the term, although word processors more often say “line spacing.”
Leading affects readability and page density. Too little can make ascenders and descenders appear crowded, while too much can make it harder for the eye to move smoothly from one line to the next. It is different from tracking, which adjusts overall character spacing, and kerning, which adjusts the space between particular letter pairs.