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.