opposition party; party out of power
Refers to a political party not currently in government; contrasts with 与党 (ruling party).
在野党は政府の提案に反対した。
The opposition party opposed the government's proposal.
次の選挙で在野党が与党になる可能性もある。
There is a possibility that the opposition party will become the ruling party in the next election.
Compound of 在野 (ざいや, 'out of office, not in power') + 党 (とう, 'party').