The root of the word is "overseer." It refers to a Bishop. It also refers to the Christian denomination, the Episcopal Church in the United States of America, which is part of the worldwide Anglican Church with its origins in the Church of England. (In Canada, it is the Anglican Church, in England it is the Church of England, in Scottland it is the Episcopal Church.) It began when Henry VIII broke off the church in England with the Roman Catholic Church (the Pope in Rome) and declared that the Church of England would be ruled from England, not Rome.
In the Episcopal Church in the Uniteed States, there is a structure of oversight which includes bishops with episcopal oversight over dioceses. Unlike the Anglican Church in other countries, Bishops are elected by conventions in each diocese with representation from clergy and laity.