Counterattack is a type of attack in the Mario & Luigi series. When a enemy attacks it can be countered by jumping or hammering the enemy. This move is in all of the Mario & Luigi series. Almost every enemy as a hint of which character it is going to attack. Counterattack power is the same as a failed attack on the player's turn. Mario and Luigi counterattack moves are using their Hammers and their jump ability, while Bowser's counterattack moves are punching and ducking in his shell. When there is a Giant Bowser battle, Bowser's counterattacks are punching like his other smaller form and fireballs. If the player has someone's attack power up, it will also boost their counterattck power. There's a badge called the Counter Badge that raises counterattack power in Mario and Luigi: Partners in Time.
Trivia
In Paper Mario: The Thousand Year Door, there is also a move somewhat like counterattacking that is called Superguarding. It also does damage to the enemy. Also, in Paper Mario: The Thousand Year Door, a status effect called Payback can deal half the amount of damage to the enemy the player is battling.