dominant-party system; one-party dominant system
A political system where one party consistently dominates elections and governance, though other parties exist and may compete.
戦後の日本は長らく一党優位政党制だった。
Postwar Japan was a dominant-party system for a long time.
一党制 is a one-party system where only one party is legally allowed, while 一党優位政党制 allows multiple parties but one consistently dominates.
多党制 is a multi-party system with no single dominant party, contrasting with the dominance in 一党優位政党制.
Compound of 一党 (one party), 優位 (dominance), and 政党制 (party system). A calque from English 'dominant-party system'.