The technical typesetting term for letter spacing. The space between letters differs between fonts and (in a properly crafted font) between different character combinations.
The subtle adjustment of letter spacing is only possible with a typesetting program like Adobe InDesign (in comparison, MS Word has the subtlety of a sledgehammer). Such adjustment is a key tool used in copy fitting but needs handling with care.
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.