A shield is an unusual piece of armor. It is worn on the arm or held in the hand and is not usable with a two-handed weapon in games that try to be "realistic" about equipment or want a balancing factor because two-handed weapons are considered powerful. In a skill-centered game, shields often have their own skill. The protection that comes from a shield may function through a separate "blocking" mechanic that differs from the mechanics of normal armor protection.
Some games allow a shield to be used to attack with a "shield bash" that is typically an additional attack in melee.
Shields typically come in a variety of sizes and, sometimes, materials.