From BR Bullpen
A dead ball situation occurs when, by rule, a hit or thrown ball can no longer be played by the fielding team.
Chapter 5 of the official rules covers the issue of "live balls" and "dead balls". In summary, a ball becomes dead in the following situations:
- When a batter is hit-by-pitch;
- When there is catcher's interference or interference by an umpire;
- When a balk is called;
- When there is interference on a batted ball;
- When a ball is illegally batted;
- When a foul ball is not caught;
- When a fielder falls out of play after making a legal catch;
- When a ball is hit out of play (including on a home run hit over the fence or an automatic double);
- When a thrown ball goes out of play as the result of an error;
- When time is called.
In many of these cases, baserunners will be allowed to advance a number of bases either fixed by rule or according to the umpires' decision. Baserunners cannot be put out while advancing in a dead ball situation, unless they commit one of the cardinal sins of baserunning: passing another runner or missing a base.
A "dead ball" according to the rules should not be confused with a "dead ball" meaning a baseball that does not travel much after being hit. It is this second definition which is meant in the expression Deadball era, for example.