In the wake of the Dan Harmon firing as Community showrunner and in the age of powerful showrunners like Mad Men's Matthew Weiner, The AV Club helpfully provides a little background on the popularization of the term:
The term “showrunner” has become so pervasive in Internet television discussion that it’s easy to forget it only entered the public consciousness in the last decade.Read the entire post here. It's well worth it.
(I trace its prominence to the 1999 book The Showrunners, by David Wild, which is most famous for following NewsRadio creator Paul Simms around for the show’s fourth season, when it seemed as if NBC might yank it at any moment.) Since there’s been some confusion over the showrunner’s role on the show as this Harmon situation has unfolded, let me define it: A showrunner is an executive producer and head writer of a program. In some cases, series will split showrunner duties between someone who handles day-to-day production details and someone who handles the writers’ room, particularly in cases where a first-time showrunner is in charge. But the showrunner is as important to a series as a director is to a film set. Regardless of who has script credit, the showrunner took a pass at that script. Regardless of who directed that episode, the showrunner is the ultimate arbiter of the series’ visual style. Every aspect of the production will bear the showrunner’s stamp in one way or another, and if he or she isn’t happy with, say, a set design, that will be changed. It’s an immensely powerful role, and in the struggle to develop something of an auteur theory for television, the showrunner has been the most frequently examined figure.
Being a showrunner is a lot of work. It might be the most difficult, wearing job in show business.