In relation to the Watergate scandal, "smoking gun" refers to concrete evidence or proof that directly links someone or something to an illegal or unethical act. It is often used metaphorically when discussing scandals or crimes.
Related terms
White House tapes: Secret audio recordings made by President Nixon in his office that were eventually subpoenaed during investigations into the Watergate scandal.
18½ minute gap: A portion of one of President Nixon's White House tapes where there was mysteriously missing audio for approximately 18 and a half minutes.
Saturday Night Massacre: The name given to the events on October 20, 1973, when President Nixon fired Watergate special prosecutor Archibald Cox and Attorney General Elliot Richardson resigned in protest.