Protection orb

"'It is the only way, Nagini,' he whispered, and he looked round, and there was the great, thick snake, now suspended in mid-air, twisting gracefully within the enchanted, protected space he had made for her, a starry, transparent sphere somewhere between glittering cage and tank."

- Lord Voldemort creating the orb for Nagini's protection.

The unnamed protection orb was a protective spell that a wizard or witch casts around a certain target to protect them from exterior attacks. It conjures a sphere around the target in question, which can levitate in the air, and is starry, transparent, making it look somewhat between a glittering cage and tank.

History of usage
Lord Voldemort used this spell in 1998, during the Battle of Hogwarts, around his snake familiar Nagini. This was to protect the snake, who was also his final Horcrux, from Harry Potter, who was aiming to kill her to rob Voldemort of his immortality. Voldemort claimed it was for her good, though in truth he did it for his own preservation.

When Voldemort thought he killed Harry, he deactivated the orb around Nagini, thinking there would be no more threat for her, a mistake that led to the snake's demise at the hands of Neville Longbottom.

Appearance

 * Harry Potter and the Deathly Hallows