The process and the result of making a composite photograph by cutting, gluing, rearranging and overlapping two or more photographs into a new image. (Wikipedia)
The process was occasionally used in advertising in the late 19th century and for novelty postcards that were popular with the Victorians (many of which were proto-surrealist). People often had pieces of their own photo portraits cut and collaged into greeting cards!
The technique was turned into a form of creative appropriation by the Dadaists and the Russian Constructivists in the 1910s.
See also, and mainly: Dadaist photomontage