party chairman; party leader
Primarily used for the leader of a political party, especially in China, Taiwan, and similar contexts. Often refers to the head of the Chinese Communist Party.
The party chairman of the Chinese Communist Party decides important national policies.
The new party chairman of Taiwan has taken office.
党首 is a general term for 'party leader' in Japanese politics, whereas 党主席 is more specific to Chinese and Taiwanese contexts.
Compound of 党 (party) and 主席 (chairman). The term is a direct translation of the Chinese title, used in Japanese to refer to the same position.