A Hidden Mickey is an image of Mickey Mouse concealed in the design of a Disney attraction or in Disney landscaping. Usually, it's in the shape of a head and ears silhouette, i.e., one large circle with two smaller circles (where the ears would be) on top of the larger one.
These are not to be confused with Decor (or Deliberate) Mickeys.
Hidden Mickeys are where there is a mickey head or shape "hidden" in the background of something. They can be anywhere. It is a lot of fun to "hunt" for them.