An attack is an action performed in the Battle Phase that results in battle. During an open game state during the Battle Phase, the turn player can declare an attack with a face-up Attack Position monster that has not already declared an attack this turn, choosing a monster the opponent controls as attack target or making a direct attack. Some cards and effects can allow monsters to attack while in Defense Position (such as "Total Defense Shogun"). Upon declaring an attack, gameplay immediately proceeds to the Battle Step (declaring an attack does not start a Chain).
If the number of monsters controlled by the controller of the attack target changes during the Battle Step, a replay is triggered, wherein the attacking player chooses a new attack target without declaring a new attack.
In the anime and manga, when a player declares an attack, the monster literally attacks the attack target. In most cases the weaker monster will just stand still while the other one destroys it, sometimes it will try to fight back, but will obviously lose unless the player somehow makes it stronger. Some attacks are also given names.