Obliviate!

Also known as the "Memory Charm" or "Memory Modifying Charm". The word obliviate is derived from the Latin word oblivi. The most common use of this charm is by the Ministry of Magic to help keep the Wizarding world a secret from Muggles.

Other uses of Obliviate in the books

Professor Lockhart accidentally obliviates himself in CoS when he uses Ron Weasley's broken wand.
In order to protect her parents, Hermione modified their memories to make them forget they had a daughter and moved them to Australia.
Memory charms were used to the witnesses of Peter Pettigrew's murder of twelve Muggles.
"If there's one thing I pride myself on, it's my Memory Charms."
- Gilderoy Lockhart

Links

HP Lexicon
Harry Potter Wiki