Expelliarmus

Expelliarmus is the textbook Disarming Charm — a simple, defensive spell built from Latin roots that mean roughly "expel the weapon." Cast at an opponent, it strikes their wand hand, breaks their grip, and sends the wand flying toward the caster. It does not kill. It does not maim. It does not even really hurt. It just ends the duel.

Harry Potter first sees the spell properly in his second year, when Severus Snape uses it to send Gilderoy Lockhart sailing across a stage in the short-lived Hogwarts Duelling Club. From that demonstration onward, it becomes the spell Harry leans on more than any other. He teaches it to Dumbledore's Army. He uses it against Voldemort in the Little Hangleton graveyard. He uses it in nearly every dangerous situation he ends up in. The Death Eaters mock him for it — even Lupin gently chides him for telegraphing his moves — but Harry's reasons are unshakable: he doesn't want to kill people, and Expelliarmus is the spell that lets him win without crossing that line.

The charm is also the unlikely linchpin of how the Second Wizarding War ends. The Elder Wand, oldest of the Deathly Hallows, transfers loyalty whenever its master is overcome. When Harry forces Draco Malfoy's wand from his hand at Malfoy Manor — using Expelliarmus, naturally — he becomes the Elder Wand's true master without ever touching it. The final confrontation at the Battle of Hogwarts hinges on that fact: Voldemort casts Avada Kedavra, Harry casts Expelliarmus, and the rebounding Killing Curse strikes Voldemort because the wand he is holding refuses to kill its rightful owner.

It is, in other words, the spell that bookends Harry's life. Snape disarms Lockhart in a kid's duelling club. Years later Harry disarms Voldemort to end a war. The wand movement is a sharp downward slash. The incantation is one word. The implications are very large indeed.