A Hidden Mickey is a representation of Mickey Mouse that has been inserted subtly into the design of a ride, attraction, or other location in a Disney theme park, Disney property, animated film, feature-length movie, TV series, or other Disney product. The most common Hidden Mickey is a formation of three circles that may be perceived as the silhouette of the head and ears of Mickey Mouse, often referred to by Disney aficionados as a "Classic Mickey".

Source: Wikipedia