Is the United States a representative republic?

Technically, no. By definition, all republics are representative in some form, since "republic" means "rule by elected officials", and elected officials are representative of the people who elected them. To use the term "representative republic" is just redundant.

While it was used in the past by at least one of the founding fathers of the United States of America, it actually doesn't mean any more than just saying "republic".

However, there are many countries that call themselves republics that are actually run by military dictatorships or religious leaders, and do not really have representative government, so the term "representative republic" could be used to emphasize countries that are truly representative.