Z button

From SmashWiki, the Super Smash Bros. wiki

The Z Button refers to the long, purple button at the top right corner of a GameCube controller, the rectangular, gray trigger located on the back of a Nintendo 64 controller, or the rectangular, white button on the back of the Nunchuk controller. It is used for rolling, grabbing, and shielding in the standard setup, but in Brawl, it can be set to a different command.