Guardian Acorn

"You've got a Guardian Acorn! It will reduce the damage you take by half!" In-game text upon finding Guardian Acorn

The Guardian Acorn is a tool found in The Legend of Zelda:Link's Awakening that is found at random after defeating a common enemy in the overworld or dungeons (much like rupees and hearts). While active, Link will only take half of the damage that an enemy attack would have incurred, and makes link temporarily invulnerable to attacks that only do a half-heart of damage. An item that works in a similar way is the Piece of Power.