What is Z trigger?

What is Z trigger? An example of this is The Legend of Zelda: Ocarina of Time, where three of the C-buttons can be assigned to secondary items and the upper C-button is used to call Navi for assistance, while the Z-trigger is used to lock focus onto enemies and center the camera behind the player.

An example of this is The Legend of Zelda: Ocarina of Time, where three of the C-buttons can be assigned to secondary items and the upper C-button is used to call Navi for assistance, while the Z-trigger is used to lock focus onto enemies and center the camera behind the player.