Lord Protector was a title of a ruler of a country that would normally be ruled by a king or a queen. During this time, the country was referred to as a Protectorate rather than a Kingdom.
In the 1650s, first Oliver Cromwell and, after his death in 1658, his son, Richard Cromwell held the title of the Lord Protector of England, Scotland and Ireland.
Richard Cromwell's becoming the Lord Protector demonstrated that it had been wrong to use the hereditary principle in the assignment of Lord Protectors. Thus, Charles II was invited to take the throne. (PROSE: The Roundheads)