1. Law: Illegal or improper alteration of, or interference with, a document or evidence; or meddling or negotiating with a witness to influence his or her testimony.
2. Quality control: Continually adjusting a process to compensate for variations in its output. If the variations are within the control limits of a stable system (and all stable systems have variations) then tampering invariably makes things worse. Only a change in the system itself (and not any ad hoc adjustment) can reduce variation and improve quality. Urge to temper (trying to do 'one's best,' but actually messing around) springs from the naïve desire to do something anything to improve results without sufficiently understanding the system, and ruins an otherwise working process. Tampering and its effects are demonstrated with the help of funnel experiment. Not to be confused with tempering.