The POW Block is a weapon found in games featuring the Mario Brothers, Mario and Luigi. Its first appearance was in the original Mario Bros. game in which it would float near the bottom center of the screen and could be hit three times before it would completely disappear. Hitting the Pow Block would count as one hit on every character on screen. Standing Shell Creepers would be knocked onto their backs, Side Steppers would either become agitated or fall over depending on how many previous hits they took, on, and Fighter Flies would be knocked over if they were touching the ground at the time. Any enemies laying on their backs would be flipped back to their feet and either Mario or Luigi would be forced to jump. Later levels in the game stopped having the POW Block appear.
POW Blocks didn't make an appearance in Super Mario Bros. but did return in Super Mario Bros. 2. In this game, the POW Block was an item that sat on the ground and could be picked up by the player's character. When thrown to the ground, the POW Block would defeat every minor enemy on screen. Unlike in Mario Bros., the POW Block only had one use before it would disappear.
Super Mario Bros. 3 briefly has the POW Block appear in a mini-game mimicking the original Mario Bros.. In this game, the POW Block had the same abilities as the original game.
POW blocks were almost exclusive to the Super Mario Bros. games but, they were also featured in Yume Kōjō: Doki Doki Panic even before it had been altered to become the better known Super Mario Bros. 2. The item functioned exactly the same in both games, except the POW block wasn't animated in the original game (in Super Mario Bros. 2 the POW letters would rotate).
In Super Smash Bros. it could be found in the special tribute stage Mushroom Kingdom. The POW Block abandons its traditional red edges and instead has blue edges. The POW Block would appear on-screen after a certain amount of time had passed. If a combatant hits the block it causes the screen to shake and 20% damage to all combatants on the ground. If the combatant is damaged enough the POW Block could throw them off screen.
In the Wii racing game Excite Truck, an item similar in appearance to the POW block grants invincibility to a truck during the race. As both games are made by Nintendo this is most likely a reference to the Mario series.
The POW block makes an appearance in The Super Mario Bros. Super Show! episode "The Pied Koopa." In this one time appearance the POW Block's effect is more like that of a Bob-omb. Mario hurles it towards a castle wall and it explodes on contact. This is the only time the POW Block appears in any of the cartoon episodes of Super Mario Bros.