noun

• (countable) The act of calling into question or challenging the accuracy or propriety of something.
• (countable, law) A demonstration in a court of law, or before another finder of fact, that a witness was ingenuine before, and is therefore less likely to tell the truth now.
• (countable, law, Britain) An accusation that a person has committed a crime against the state, such as treason.
• (countable, law, chiefly US) The act of impeaching or charging a public official with misconduct, especially if serious, often with the aim of having the official dismissed from office.
• (uncountable) The state of being impeached.
• (uncountable, archaic) Hindrance; impediment; obstruction.