55-year system is a literal translation of the Japanese 55-nen taisei (55年体制).

This phrase refers to the large-scale political realignment which took place in Japan in 1955, in which various factions and smaller parties coalesced into several larger political parties, most prominently the conservative Liberal Democratic Party (LDP) and the liberal Social Democratic Party (SDP).

These two parties would be the dominant players in national politics for the next thirty or forty years, with the LDP being the chief ruling party and the SDP being the chief opposition party. Support for the Japanese Social Democratic Party has slowly ebbed over the years, and the once unquestionable grip of the Liberal Democratic Party has lately come to be loosened as well, as witnessed in the 2003 elections.