The ax (or axe) 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 ax, 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 ax is the only way they can defeat Bowser.
Later on, an ax appears as Treasure J in Course 29 of Wario Land: Super Mario Land 3. Another ax plays a similar role in Wario Land 3, where it serves as Out of the Woods's Gray Treasure. With this ax, Wario cuts down a tree blocking him from The Peaceful Village and The Vast Plain.
Due to the nostalgia of Super Mario Bros., the ax (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 ax. 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 ax is replaced by a Skull Switch in that game's Bowser battles (same with New Super Mario Bros. Wii, but with a ! Switch). The ax has a true return after 27 years in New Super Mario Bros. U, where, similar to Super Mario Bros, it destroys Bowser's bridge. However, the player doesn't actually utilize the ax. Rather, the player has to use a ! Switch to activate it.