noun
party leader; leader of a political party
Refers to the head or leader of a political party. It is not used for the host or leader of a social party.
野党の党首が記者会見を開いた。
The opposition party leader held a press conference.
党首は選挙後に辞任を表明した。
The party leader announced his resignation after the election.
Means a political party itself; 党首 is the person who leads one.
A Sino-Japanese compound: 党 means political party or faction, and 首 means head or leader. Together they mean the head of a party.