In baseball, middle relief pitchers (or "middle relievers") are relief pitchers who commonly pitch in the 6th or 7th innings or in situations where several innings worth of work is required (such as extra innings or when the starter is pulled early in the game). The 8th and 9th innings are generally reserved for specialty pitchers (such as left-handed specialists, setup pitchers or closers who earn saves and holds) but middle relief pitchers may work these innings as well, especially if the game is not close.
Middle relievers have two goals once they are brought into the game to replace an ineffective starter. The more hopeful aim is that they will pitch more effectively than did the starter, thus increasing the team's chances of hitting its way to a comeback victory. The more realistic goal is for them to simply compile several innings pitched as a way to protect more valued relief pitchers from fatigue and overuse, thus keeping them fresh and available for use in crucial game situations (e.g. slim lead). In the Major Leagues, middle relief pitchers are often minor leaguers who are being auditioned for more important roles with the club. Otherwise, the role is usually reserved for pitchers who have been unsuccessful in other roles on the staff.
Rudy Seanez is an example of a middle reliever who has had a long and successful career.