noun
leading party; dominant party
Used in parliamentary contexts to refer to the party with the most seats or greatest influence.
選挙の結果、自民党が第一党となった。
As a result of the election, the Liberal Democratic Party became the leading party.
野党 is an opposition party, whereas 第一党 can be either a ruling or opposition party depending on the election outcome.
Compound of 第一 (first, number one) and 党 (party).