Any of countless sets of three
circles, slightly overlapping, with the top two usually smaller than the bottom one, that are hidden throughout the
Disney theme parks and other Disney productions. Such a
configuration does, of course, form the silhouette of
Mickey Mouse's head, the icon of
the Walt Disney Company.
Hidden Mickeys are to Disney's imagineers and animators as easter eggs are to programmers. They put them anywhere they can find room without being conspicuous, and some are incredibly hard to find. Some people make it a point to find as many as they can (either via sharp-eyed searching or based on lists found on the Internet) when they visit a Disney park.
The exact definition of a Hidden Mickey varies somewhat... some people claim that any appropriate arrangement of similar shapes qualifies, while others insist that all three be perfect circles and that the ears be smaller than the head. As such, it's often hard to know if what you're seeing is really a Hidden Mickey or just a coincidence.
The definitive web site for Hidden Mickeys is at www.hiddenmickeys.org.