Any penalty that is called or assessed after the whistle has been blown to declare the play dead. Dead ball fouls are assessed from the end of the play, rather from the previous line of scrimmage or spot of the foul. The most common dead ball fouls are unsportsmanlike conduct and unnecessary roughness, since they take place during post-play moments.
Dead ball fouls are considered the easiest to avoid and the biggest game changers since they don't take place during the action on the field. Most dead ball fouls are heat of the moment penalties that occur due to a reaction to other events near the whistle. Since the penalty is assessed after the play, a dead ball foul can either set an offense back drastically or wildly enhance a team's chances of scoring. Most dead ball fouls fall into the unsportsmanlike conduct or unnecessary roughness categories, and both of those are fifteen yards from the end of the play.