Unlocking Charm

Alohomora is a charm that opens locks. It is also able to open doors locked by Colloportus. Some unknown spells make locked doors impervious to Alohomora.

Known Uses
Hermione Granger used this spell to open a forbidden entrance on the Third Floor Corridor of Hogwarts Castle in 1991. She also used this on Professor Filius Flitwick's office window to rescue Sirius Black in 1994.

The Janus Thickey Ward at St. Mungo's Hospital for Magical Maladies and Injuries can only be entered by using this spell. Healers and visitors may enter, but patients are unable to get out.

The door guarding the Philosopher's Stone in 1991 was protected against Alohomora; a winged key was necessary to open it. A single door in the Department of Mysteries was impervious to this spell as well. It is the door which presumably conceals the room where the Ministry of Magic studies the power of Love.

Hermione used this to open Regulus Black's bedroom in 12 Grimmauld Place.

Etymology
From the West African Sidiki dialect used in geomancy meaning: Friendly to thieves as stated by J.K. Rowling in testimony during the WB and JKR vs. RDR Books.

Behind the scenes

 * Despite the fact that Dolores Umbridge's office at Hogwarts was (assumed) impervious to this spell in the novel, Harry Potter uses the spell to open it before attempting to use the Floo Network in the film adaptation of Harry Potter and the Order of the Phoenix.

Appearances

 * Harry Potter and the Philosopher's Stone
 * Harry Potter and the Philosopher's Stone (film)
 * Harry Potter and the Philosopher's Stone (video game)
 * Harry Potter and the Chamber of Secrets (video game)
 * Harry Potter and the Prisoner of Azkaban
 * Harry Potter and the Prisoner of Azkaban (video game)
 * Harry Potter and the Goblet of Fire (video game)
 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Order of the Phoenix (film)
 * Harry Potter and the Deathly Hallows

Notes and references
Alohomora Алохомора