A fold or a folding back. A reply to an answer; a rejoinder. Law The plaintiff's response to the defendant's answer or plea. An echo or reverberation. A copy or reproduction. The act or process of duplicating or reproducing something. Biology The process by which genetic material, a single-celled organism, or a virus reproduces or makes a copy of itself: replication of DNA. In scientific research, the repetition of an experiment to confirm findings or to ensure accuracy.