noun
leading opposition party; largest opposition party
Refers to the opposition party with the most seats in a legislature; used in political contexts.
野党第一党は、次の選挙で政権交代を目指している。
The leading opposition party is aiming for a change of government in the next election.
国会では、野党第一党が予算案に反対した。
In the Diet, the largest opposition party opposed the budget bill.
Compound of 野党 (opposition party) + 第一 (first) + 党 (party).