It's not a true one-party dictatorship for the fact that people still have a choice at the ballot box, but the victories are clearly weighted in favor of a single dominant party. The icing on the cake is that the dominant party either colludes with or is a subsidiary of major private and business interests.
In modern parlance, we would call collusion between state and corporate power corporatism. The generation that fought World War 2 called it fascism.
The only difference is this fascism actually finds the presence of elections and a loyal opposition party useful. It makes people think they're still free and that they have a choice. As long as people are kept in the thrall of such an illusion, the business interests that control this structure will face less chance of resistance from the workers.
Care must be taken to preserve the fragile veneer of democracy. Blatant rigging of elections would invite a violent response. The leadership in Iran found out the hard way when they so blatantly rigged their parliamentary elections of 2009 that many people called foul. They gained back control of the situation by beating the population into submission, but people inside Iran know they're dealing with a dictatorship now. Their eyes are open.