To quote John August (Frankenweenie, Charlie and the Chocolate Factory, Charlie's Angels...):
The highest ranking producer is the showrunner, the man or woman ultimately responsible for the creative direction of the series.
Showrunner is a function, not a title...
... this person is credited as an Executive Producer. In many cases, he or she has “Created by” credit on the series
- Note: this doesn't mean that every Executive Producer is a "showrunner"
(a leading actor is credited as an actor, but that doesn't mean every actor is a leading actor)
Also of note:
Because scripted television is run by writers, the majority of producers you see listed are writers.
Ken Levine (*M*A*S*H*, Cheers, Frasier,...) has this to say:
[The Showrunner] oversees the entire production and essentially provides the voice and creative direction of the show.
He hires the writing staff, the crew, the directors. He is in charge of the show’s budget.
He approves and breaks the stories, assigns them, rewrites them, and decides when the scripts are ready for distribution.
He does all of the casting. Deals with the network and studio. Approves sets, wardrobe, music, ...
He also oversees the editing. The editor and director put together their first cut and then the showrunner is in control. He can change that cut at will, if the show is long he determines what gets cut.
He then supervises post production – sound, color correction, music.
How the hell does he have time to do all of that?
He doesn’t. And that’s the real art of showrunning.
So even though a Showrunner is basically responsible for everything, he doesn't actually have to do everything. He can of course appoint people to do things for him.
When Did People Start Saying “Showrunner”?
Most likely, the late 1980s.
The rise of the term “showrunner” tracks with the rise of writers in television.
- Before TV became a “writer’s medium,” it was a studio’s medium. Throughout the ’50s and ’60s, studios controlled all facets of production, from conception onward. Writers were mostly contract workers and did not generally participate in crafting the vision of the show. Studios only allowed experienced executives to pitch new shows, and these executives typically stayed on as the chief producers.
As writers became more critical to keeping shows on air, they were given more production responsibilities. Soon, studios even allowed writers to create new shows—and then to stay on to shape the shows they envisioned. The writer-executive producer was born.
But in an era of producer credit proliferation, those on set needed a new, shorter term to separate the person making final creative decisions from other producers.
“Showrunner” became natural shorthand for the person who literally runs the show.