Sol

In Twilight Princess, a Sol is an object that lights the Twilight Realm, similar to the sun of the World of Light. It is made of pure power and provides energy and life to the citizens of the Twilight Realm. The light of the Sols, later in the game, fuses with the Master Sword to create a "Light Sword."

Bringing the two Sols to the plaza in front of the Palace of Twilight enables their light to be infused into the Master Sword. Placing a Sol on special pedestals can cause platforms and steps to appear. Bringing a Sol (or the light-infused Master Sword) near to a cursed Twili will cause the Twili to revert to its original form.

Trivia

 * The word "Sol" originates from the Latin word for the Sun. Sol and its derivatives are are still used in a number of modern languages.
 * In Norse Mythology, Sól is the goddess of the Sun.