Definition of "showrunner"

Those so-called showrunners, the thousand or so people who share both writing and managerial duties on television shows, have already been squeezed by conflicting demands from guild leaders and company employers.

2007 November 3, Brooks Barnes, “With Resolution Unlikely, Writers Guild Sets Strike for Monday”, in New York Times‎

The person at the apex of the production triangle is the showrunner: in short, the boss.

2013, Bethany Rooney, Mary Lou Belli, Directors Tell the Story: Master the Craft of Television and Film Directing, page 66