A collage is a work made by gluing different materials, such as photographs, newspaper cuttings, paper and other objects, onto a surface to create a new image. The word comes from the French "coller", which means "to glue".
Each piece brings its own meaning, and together they create new and often surprising meanings. Pablo Picasso and Georges Braque made collage an art form around 1912, during Cubism. It was later used by Dada, Constructivist and Surrealist artists. Collage is closely related to photomontage, which combines photographs specifically, but the two terms do not always mean the same thing.
Do you have a better definition? Share it in the comments.