Interference and Obstruction Rulings
Interference is a broad rule book term that refers to a number of illegal actions that occur during a contest to change the course of the game. Interference can be committed by the team at the plate or the team in the field, as well as by players not even in the game at the time, an umpire, a fan, or another individual not associated with the team.
One key distinction between interference and obstruction: Interference is defined as a violation of either the offense or the defense; obstruction can only be committed by the defense.
Offensive interference is the most common infraction and refers, according to the Official Rules of Major League Baseball, to "an act by the team at bat which interferes with, obstructs, impedes, hinders or confuses any fielder attempting to make a play." An umpire can call a batter, batter-runner or runner out for offensive interference when he engages in any of the above behavior against the defensive team, sending any other runner to the last base that the umpire determines he occupied before the call was made.
Catcher's interference occurs when a catcher interferes with a batter's ability to hit a pitch. This happened when the batter swings and strikes the catcher's glove or portion of the catcher's body, usually during steal attempts when the catcher has inched too close to the batter in order to prepare to make a throw. When catcher's interference is called, the batter is awarded first base. Those runners on base who are forced by the batter to advance are awarded the next bag. If a runner had been attempting to steal, he is safe.
Spectator interference occurs when a fan or another individual not associated with the team - such as a bullpen attendant or a batboy - alters a play in progress. Such interference most typically occurs on foul balls hit into the first row of the stands that a fielder would have had a chance to catch if the fan did not prevent him from doing so.
Whether such incidents are deemed interference depends on whether the umpire determines that fielder could have caught the ball over the field of play. If the fielder reaches into the stands and is hindered in making a catch, no interference is called. If the fan reaches over the railing and thwarts the fielder's attempt at a play, interference is called. The umpire will award any outs or bases depending on what he believes had the interference not occurred.
Obstruction is the act of a fielder who, while not in possession of the ball and not in the act of fielding it, impedes the progress of any runner on the base paths. According to Official Rules of Major League Baseball, "If a fielder is about to receive a thrown ball and if the ball is in flight directly toward and near enough to the fielder so that he must occupy his position to receive the ball, he may be considered 'in act of fielding a ball'." The umpire may use his discretion to determine if a fielder is engaged in such an act. A player is no longer considered to be actively fielding a batted ball after he has made an unsuccessful attempt to do so, and any conduct that impedes an offensive player after such an attempt qualifies as obstruction of that runner.