The phrase "preexisting material" refers to something that already exists or is already present before a particular point in time or situation. It means something that is already there or has already been made. Full definition
His use of preexisting materials not traditionally used in art making, varied painting surfaces and modes of construction were pivotal in the reemergence of painting in the United States. (theglasshouse.org)
God did not have to work from preexisting material that resisted his purposes. (firstthings.com)
Since the mid 1970s, Prince has incorporated preexisting material into his photographs, paintings, and sculptures, including advertisements, car hoods, cancelled checks, and celebrity head shots. (levygorvy.com)