a famous political scandal in the US in the early 1970s that caused President Nixon to leave his job before Congress could impeach him (=charge him with a serious crime). It was discovered that Nixon had agreed to an attempt to obtain information about the Democratic Party's plans for the next election, by secretly going into their offices in the Watergate hotel in Washington D.C. Nixon later tried to prevent this information from being discovered. The Senate asked to hear recordings that Nixon had made of conversations in his office, but when they received them, parts of conversations seemed to have been deliberately removed. These recordings became known as the 'Watergate tapes'.The Watergate Affair shocked people in the US, and made them less willing to trust their political leaders. Because of these events, other political scandals are often given a name ending in '-gate', for exampleIrangate.
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.