1. The act of altering a thing; esp., the act of illegally altering a document or product, such as written evidence or a consumer good. See Model Penal Code §§ 224.4, 241.8; 18 USCA § 1365.
2. The act or an instance of engaging in improper or underhanded dealings, esp. in an attempt to influence. • Tampering with a witness or jury is a criminal offense. See WITNESS-TAMPERING; OBSTRUCTION OF JUSTICE; EMBRACERY.