A charm which hides the true, magical nature of something.
The thirteen witches and wizards who came to collect Harry from number four, Privet Drive used Disillusionment Charms on him in book 5.
When a Disillusionment Charm is used on you, it feels like something cold is running down your back from your head. When it is lifted, you feel something warm running down your back.
Hermione suggested using a Disillusionment Charm when planning the visit to Godric's Hollow.
One of the ways to create an Invisibility Cloak is to imbue a travelling cloak with a Disillusionment Charm.