An antigen refers to a substance that triggers an immune response in the body. It is not the result, opposite, or residue of an antibody. Instead, an antigen is the stimulus that leads to the formation of antibodies. When the body comes into contact with an antigen, the immune system recognizes it as foreign and produces specialized proteins called antibodies. These antibodies bind to the antigen, neutralize it, and aid in its elimination from the body. This immune response is crucial for defending the body against pathogens like bacteria or viruses. Therefore, option 3, "the stimulus for antibody formation," is the correct explanation for what an antigen is.