Dominant party system is the system in which one party is so dominant that other parties are, for practical purposes, irrelevant.
No, it is not same as one-party system. It is nearly same to the one-party system, but differs in that, there exists a number of parties. In India until 1990, the Congress Party maintained its predominant position, barring 3 years, as the ruling party at the national level.
In Japan, also the Liberal Democratic Party, until 1993, has been the sole governing party of the country for more than • three decades though a number of parties are represented in the Diet. This system also existed in East Germany, Poland 1 and Czechoslovakia, where one-party forced other parties into an electoral pact that voters had no other choice,but to vote for the candidates of the coalition or to abstain.