Muggles' Guide to Harry Potter/Magic/Dissendium

Overview
Dissendium is a single-purpose spell; used on the statue of the hump-backed, one-eyed witch in the third-floor corridor at Hogwarts, it causes her hump to hinge open

Extended Description
When her hump is open, it reveals the end of a passageway that goes to the basement of Honeyduke's Sweetshop in Hogsmeade. Harry uses this passageway to leave the castle without permission, in Harry Potter and the Prisoner of Azkaban; the twins have apparently been doing the same for years, as the Marauders did many years before them; and it seems Sirius Black is also using this as a means of getting into the castle.

Analysis
A certain number of spells seem to have a single use at a single location, much like passwords; rather than being available for general use, they seem to be tied to specific locations or artifacts. Dissendium appears to be one such; similar to Alohomora, the unlocking spell, but tied to this one statue. The Marauder's Map is similarly opened and hidden with passwords that are not, strictly speaking, magical at all, though they have magical effects.