The Lord Protector was the head of state during the brief period of the republic or "Commonwealth" in Great Britain and Ireland. Oliver Cromwell and afterwards his son Richard Cromwell were the Lords Protector of England, Scotland and Ireland during the period 1649 to 1659.
The title of Lord Protector was not an invention of Cromwell's, however. It had previously been used by princes or other nobles exercising a regency while the heir apparent was still a minor. Notable examples of this are Richard, Duke of Gloucester who was Lord Protector to the 'Princes in the Tower' and Edward Seymour, Duke of Somerset, who was Lord Protector to the young Edward VI. It has not been used for any regent since the Restoration and it's highly unlikely that the title will ever be revived in the future.
