Refers to a political party other than the two largest or dominant parties in a system, often used in discussions of elections and parliamentary politics.
In the election, the third party increased its seats.
Voters tired of the two major parties' conflict shifted to a third party.
Refers to the two major parties in a political system, whereas 第三政党 is any party outside those two.
A minor or minority party; 第三政党 often implies the third-largest party, while 少数政党 is a broader term for any small party.
Compound of 第三 (third) and 政党 (political party). The term is a straightforward modern political term with no obscure historical derivation.