Rules 6.05 & 6.09 - Uncaught third strike

In the Majors division, an uncaught third strike(commonly but erroneously referred to as a dropped third strike)occurs when the catcher fails to cleanly catch a pitch for the third strike. Any pitch is considered uncaught if the ball touches the ground before being caught or is dropped after being caught. The distinction is important as a cleanly caught ball that has skipped in the dirt is an uncaught ball for purposes of this rule. The specific rules concerning the uncaught third strike are Rules 6.05(b) and 6.09(b).

On a dropped third strike with no runner on first base or in any situation with two outs, the batter immediately becomes a runner. The strike is called, but the umpire does not call the batter out. The umpire may or may not verbalize "no catch". The batter may attempt to reach first base and must be tagged or thrown out. With two outs and the bases loaded, the catcher who fails to catch the third strike may step on home plate for a force-out or make a throw to any other base for the force. 

If at the time of the strike three pitch, first base is occupied with fewer than two outs, the batter is out and cannot become a runner. This is to avoid having the defense deliberately drop a third strike pitch to set-up a double or even triple play.

Regardless of the outcome of an uncaught third strike, the pitcher is credited with a strikeout. Because of the dropped third strike rule, it is possible for a pitcher to record more than three strikeouts in an inning.

In Tee Ball, AA and AAA, the dropped third strike does not apply and the batter is out on any called or swinging third strike regardless of whether it is caught by the catcher.