The POW Block, also occasionally referred to simply as "POW", is an item that appears in Mario Bros. (the arcade version and its ports; not the Game & Watch one). When hit, it damages all enemies in the screen, but only those that were on the ground. It performs the same function in Super Mario Bros. 2, but it can only be used once as the player has to throw it onto the ground to make a POW.
In Super Mario World 2: Yoshi's Island, if Yoshi hits a POW block, all nearby enemies turn into stars. POW Blocks were most commonly won in Bonus Challenges, where they could be used anywhere afterwards, being called Anywhere POWs.
In the Paper Mario series, the POW Block is an item that does two damage to all enemies (but much more in Super Paper Mario if the remote had been shaken with each corresponding prompt). It penetrates defense and makes enemies with shells (such as Koopa Troopas) turn over. It does not affect flying enemies, however, unless they are on the ceiling.
The POW Block also appears twice in the Super Smash Bros. series. In Super Smash Bros., the POW Block appears randomly on the Mushroom Kingdom stage. Attacking it damages everyone on the ground and sends them skyward. In Super Smash Bros. Brawl, it appears in the Mario Bros. stage. Unlike in Super Smash Bros., though, this POW Block doesn't damage anyone, but merely sends them skyward somewhat. Like in the original Mario Bros., it is mainly used to knock over Shellcreepers and Sidesteppers.
The POW Block is a new item in Mario Kart Wii. When dispatched by a player, all racers in front of the user will find POW Blocks above their karts. They must shake the Wii Remote on the third strike or be in the air to avoid the POW Block or they will go into an explosive spin, drop their items, and lose speed. After Mario Kart Wii, this item doesn't return in any future Mario Kart games.
In New Super Mario Bros. Wii, POW returns as a item. Like in Super Paper Mario, Mario, Luigi, or one of the Toads can throw POW and KO all enemies around and drop coins from the sky. It first appears in world 1-2.