The axe (or ax) is an item in the castle levels of the original Super Mario Bros. Mario or Luigi can defeat Bowser or his duplicates by touching the axe, which cuts the support of the bridge Bowser is standing on and sends him falling into the lava below. When Mario and Luigi do not have the Fire Flower, using the axe is the only way they can defeat Bowser.
Due to Super Mario Bros., the axe (which plays a major role in its boss battles) has managed to earn Bowser-related appearances to this day. For instance, in Bowser's court in Mario Hoops 3-on-3, the basketball hoop's pole is a giant axe. It also appears in Mario & Luigi: Superstar Saga, where it cameos alongside a paper duplicate of Bowser in a Super Mario Bros.-reminiscent section of Bowser's Castle. In New Super Mario Bros., the axe is replaced by a Skull Switch in that game's Bowser battles (same with New Super Mario Bros. Wii, but with a ! Switch). The axe has a true return after 27 years of absence in New Super Mario Bros. U, where, similar to Super Mario Bros, it destroys Bowser's bridge and sends Bowser into lava. However, the player doesn't actually utilize the axe. Rather, the player has to use a ! Switch to activate it.
The axe also appears in Super Mario Maker, as the goal for Castle-style courses. Its appearance is different depending on the game style, but it has the same function as usual; touching it destroys the bridge under it and ends the level.