A dead ball typically occurs in basketball after the ref calls a foul or when a foul has been shot, another one is going to be made, and/or the ball has gone out of bounds. The term of a “dead ball” refers to the ball not being playable. The NBA has defined that this is a good time for player substitutions to be made, especially if a player is injured during play.
However, the answers given here aren’t quite right. A dead ball is typically called by the ref, yes, but if it rebounds and doesn’t go out of play, that’s not a dead ball. Instead, the ref is left to call it if he sees something that would make it a foul instead of just a rebound.