A multiball is a condition or mode in a pinball game in which multiple pinball are in play at the same time. The first intentional use of a multiball was in Bally's Balls-A-Poppin machine in 1956. Williams' Firepower was the first solid state pinball machine to feature a multi-ball.
Multiballs are commonly obtained by "locking" three balls. The amount and the way of locking the balls varies by table. While 3 balls is a common amount of balls for a multiball, there are many variations that use 2 balls or more than 3.
In many games the goal during a multiball is to hit certain targets, ramps, or lanes to obtain a jackpot.
Balls-A-Poppin by Bally was the very first pinball machine to incorporate a multiball feature. This table was first released in 1956.
Firepower by Williams was the very first solid state pinball machine to feature a multiball.
Apollo 13 features a 13 ball multiball. On top of having a 13 ball multiball, Apollo 13 also features a more standard 3 ball multiball mode. The 13 ball mode is the highest ball count of any pinball machine.