SpellsCharm
Protego
Protego is the Shield Charm. It blocks or turns aside many jinxes and minor curses. Harry learns it with Dumbledore’s Army and later teaches others to cast it. It does not stop the Killing Curse in the scenes the books show.
In the books
Harry Potter and the Order of the PhoenixChapter: Dumbledore's Army
The Shield Charm is part of what Dumbledore’s Army practices in the Room of Requirement. This page marks that teaching year rather than a single first sentence.