The Term Roman Catholic has two meanings -one used by outsiders and one used by Catholics themselves. First the outsider term refers to any Catholic as the Bishop of Rome- is the supreme head of the Church on earth. read more
Even the Catholic Church sometimes refers to itself as the "Roman Catholic Church", especially if there is any danger of confusion, or of offence. This Wikipedia article gives some history of the usage of the term Catholic. read more
However, the Roman Catholic Church is so called because it has its base in Rome, Italy, in the independent state (the smallest country in the world) within Rome called the Vatican City. read more