This is an entry in our Disneyland 101 Glossary: Disneyland Resort definitions in under 101 words, from Mouse Brief.
A Hidden Mickey is a subtle visual reference to Mickey Mouse found around Disney parks and resorts. These are little surprises Imagineers have worked into larger designs. For example: a three circle outline of Mickey’s head (one large circle topped by two smaller ones) incorporated into a busy carpet design. The three-circle depiction is the most common, but Mickey can also be rendered in profile or full body silhouette. An overt depiction of Mickey, like a large Mickey-shaped jack-o-lantern is not categorized as hidden, but a cluster of three sneakily proportioned pumpkins placed in a larger fall scene could be.