Dead ball

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:

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.