noun
written resignation from a political party; notice of resignation from a party
Refers to the formal document submitted when a member leaves a political party. Often used in news reports about politicians.
議員は離党届を提出した。
The lawmaker submitted a notice of resignation from the party.
離党届が受理された。
The resignation from the party was accepted.
Compound of 離党 (りとう, 'leaving a party') and 届 (とどけ, 'notice, report'). The kanji 届 means 'deliver' or 'report', and the word follows standard Sino-Japanese word formation.