A commoner, in British law, is someone who is neither the Sovereign nor a peer. Therefore, any member of the Royal Family who is not a peer, such as HRH Prince William of Wales or HRH The Princess Royal, is a commoner, as is any member of a peer’s family, including someone with a courtesy title, such as the Earl of Arundel (eldest son of the Duke of Norfolk) or Lady Victoria Hervey (a daughter of the 6th Marquess of Bristol).
Traditionally, members of the House of Commons were commoners and members of the House of Lords were peers, although peers whose only titles are in the Peerage of Ireland have been able to stand for election to the House of Commons for centuries. Since the House of Lords Act 1999, which excluded most hereditary peers from the House of Lords, most hereditary peers can now stand for election to the House of Commons, even though they are not commoners. For example, the 3rd Viscount Thurso and the 3rd Viscount Hailsham are currently members of the House of Commons.
The term is frequently used to refer to those not of royal blood, but this usage is incorrect.