The technical typesetting term for line spacing. The name derives from the practice at the time of metal typesetting to insert lead strips between lines of type to increase the line spacing.
The normal leading for type is 1.2 times the point size (hence 12 points of leading for a 10-point font) and is often expressed as Baskerville 10/12 (for instance). However, this convention is often broken by typesetters (e.g. when colliding title lines together for artistic effect or making smaller body type more readable by increasing the leading).
Getting right the amount of ‘air’ between lines of type has an important effect on readability.
This definition is extracted (and expanded on) from the book Getting Published: A Companion for the Humanities and Social Sciences by Gerald Jackson and Marie Lenstrup. It was first referred to in the blog Getting Published in a post on calculating book length.