Dead Ball Foul
A foul that occurs between when a play is whistled dead until the ball is snapped for the next play, or a taunting foul that occurs at any time.
Encroachment
When a defensive player enters the neutral zone and contacts an offensive player or the ball prior to the snap, or interferes with the ball during the snap, it is encroachment and the play is dead immediately.
Live Ball Foul
A foul that occurs after the ball is snapped until the play is whistled dead.
Officials
The third team on the field during an NFL game, the officials are the seven men or women that enforce the rules of the game. Each official has specific responsibilities that account for all players, areas on the field and aspects of an NFL game. Learn the NFL officials’ roles and responsibilities.
Offside
A player is offside when any part of his body is in the neutral zone or beyond the free kick line before the ball is put in play, resulting in a five-yard penalty.
Penalty
When either the offense or defense violates the rules of the game, they are assessed a penalty. The teams and fans know a penalty has been called when an official throws a yellow flag on the field.
Unsportsmanlike Conduct
Any non-contact act contrary to the generally understood principles of sportsmanship. Unsportsmanlike conduct results in a 15-yard penalty.
Source: NFL.com