Type: Charm
Pronunciation: dih-SEN-dee-um
Description: Although the only known canonical effect is to open secret passageways, it's possible, based on its use in 1997, that it opens things in general.
Seen/Mentioned: Used multiple times in 1993 to open the statue of Gunhilda of Gorsemoor, then again four years later in a failed attempt to open Salazar Slytherin's Locket.
Etymology: There are numerous suggestions.
Notes: This may not be a spell at all in the strict sense but a password; however, when used for the statue of the hump-backed witch, one must tap the statue with their wand, indicating that it is in fact a spell.