The four Major tournaments, also incorrectly called the Grand Slams, are the most important tennis events of the year in terms of world ranking points, tradition, prize-money awarded, and public...
An open circuit is called open because the circuit has an opening, a disconnection. In other words, there is an open space in the circuit in which the electrons cannot move through.
People in the golf world swap back and forth but I think "THE" Open sounds better to most mainly it's because it was the first major, and was held for 35 years before the next golf major came to be...