A dead ball is a ball that is out of play. The ruling of a dead ball halts the game and no plays can legally occur until the umpire resumes the game, though baserunners can advance as the result of acts that occurred while the ball was live. Dead balls are frequent occurrences during a game, and the dead-ball period typically does not last long before the ball is put back into play.
Dead balls most frequently occur when a batted ball becomes a foul ball or a fair ball is hit out of the playing field. Other common instances in which the ball is ruled dead include a batter being hit by a pitch, a balk, an illegal collision at home plate, obstruction of a baserunner, interference with a fielder's right of way, spectator interference, a batter or runner being granted time out by the umpire and a fair batted ball striking an umpire or runner.
If a fair ball gets lodged in the outfield wall padding -- or the ivy, in the case of Wrigley Field -- it is a ground-rule double. On all ground-rule doubles, the ball is dead, the batter-runner goes to second and all additional runners are permitted to move up two bases from the one they occupied at the time of the pitch.
Watch: Salvador Perez makes a barehanded play on a popup, but the home-plate umpire rules the play dead because the ball struck a wire.