Wrackspurt

"A Wrackspurt... They're invisible. They float in through your ears and make your brain go fuzzy, I thought I felt one zooming around in here."

- Luna Lovegood to Harry and Neville while onboard at Hogwarts Express in 1996.

A Wrackspurt is an invisible creature which floats into a person's ears, making his/her brain go fuzzy. Wrackspurts are one of the many strange creatures that Luna Lovegood believes exists though not many others do.

Behind the Scenes
In the film version of Harry Potter and the Half-Blood Prince, wrackspurts can be seen by using spectrespecs. In this way, Luna is able to locate Harry under his Invisibility Cloak after he was petrified by Draco Malfoy.