Alohomora

Alohomora — known formally as the Unlocking Charm — is one of the oldest spells in the wizarding canon. The incantation is generally believed to derive from a West African Sidiki dialect phrase meaning roughly "friendly to thieves," which the Hogwarts curriculum has politely never advertised. The effect is exactly what the name suggests: cast at a locked door, it pops the lock open with a soft click.

Hermione introduces the spell to Harry and Ron in their first year, when the trio runs into a locked door in the third-floor corridor and discovers the three-headed dog standing on the trapdoor to the Stone. From that moment forward, Alohomora is the trio's first response to any locked door they encounter, which is a surprising number of locked doors.

The charm has limits. It will defeat ordinary muggle locks (padlocks, bedroom doors, school cupboards) and most basic magical locks (interior Hogwarts doors, classroom cabinets, simple home wards), but it does not work against doors that have been deliberately warded against it. The third-floor Stone door is locked but unwarded, so Alohomora opens it; the door at the end of the Department of Mysteries is warded and refuses to budge for any spell at all. Gringotts vaults, Hogwarts's most sensitive rooms, and any door a wizard genuinely wants to keep closed will all sit perfectly still no matter how many times you cast the charm.

There are stronger counter-locking spells in the curriculum — the Anti-Alohomora Charm, the Colloportus Charm, various Bedazzling Hexes used as ward overlays — and the resulting magical arms race between locks and counter-locks is older than the school. Alohomora wins most of the time, which is precisely why ambitious doors keep escalating.