Shield
from Zeldapedia, the Legend of Zelda wiki
Shield is the term given to a type of personal armor that Link can carry alongside his sword. It is commonly used to block enemy attacks and projectiles when simply dodging is not feasible/possible. Link has wielded at least one kind of shield in every zelda game to date, and it is one of the most recognizable and useful items in his inventory. In some games, such as the original, Link starts out with a relatively weak shield, while in others, such as Ocarina of Time, he must aquire his first shield from elsewhere. It is also possible to lose an aquired non-unique shield by being swallowed by a Like Like.
What the shield protects against generally depends on the relative size of the shield, and the material out of which it has been made. Usually, a wooden shield can protect Link against minor hazards such as spears, arrows and small rocks, but cannot protect against fire-based attacks without catching fire and burning up. Metal shields are more sturdy and in most cases fire-resistant. In some games, Link can aquire a specialized mirror shield, whose reflective surface can reflect light and more powerful forms of magic.
Notably, in Twilight Princess, one of Link's Hidden Skills, the Shield Attack, allows him to bash foes with his shield to stun them temporarily, or bounce enemy projectiles back on the assailant.

