Minister of the Crown is a formal constitutional term used in Commonwealth realms to describe a minister to the reigning sovereign or their viceroy. The term indicates that the minister serves at His/Her Majesty's pleasure, and advises the sovereign or viceroy on how to exercise the Crown prerogatives relative to the minister's department or ministry.
In Commonwealth realms, the sovereign or viceroy is formally advised by a larger body known as a privy council or executive council, though, in practice, they are advised by a subset of such councils: the collective body of ministers of the Crown called the ministry. The ministry should not be confused with the cabinet, as ministers of the Crown may be outside a cabinet.
Ministers of the Crown in Commonwealth realms have their roots in early modern England, where monarchs sometimes employed "cabinet councils" consisting of Ministers to advise the monarch and implemented his decisions. The term Minister came into being as the sovereign's advisors "ministered to", or served, the king. Over time, former ministers and other distinguished persons were retained as peripheral advisers with designated ministers having the direct ear of the king. This led to the creation of the larger Privy Council, with the Cabinet becoming a committee within that body, made up of currently serving Ministers, who also were heads of departments.
During a period from the accession of King James VI of Scotland to the throne of England in 1603 and the unification of Scotland and England in 1707, the two entities separate "countries" in personal union through the one monarch who was advised by two separate sets of Ministers of the Crown for each country. As the English overseas possessions and later British Empire expanded, the colonial governments remained subordinate to the imperial government at Westminster, and thus the Crown remained ministered only by the Imperial Privy Council, made up of British Ministers of the Crown. When Canada became a Dominion in 1867, however, a separate Canadian Privy Council was established to advise the Canadian Governor General on the exercise of the Crown prerogative in Canada, although constitutionally the viceroy remained an agent of the British government at Whitehall.
After that date, other colonies of the empire attained Dominion status and similar arrangements were made. Following the passage of the Statute of Westminster in 1931, however, the dominions became effectively autonomous kingdoms under one sovereign, thus returning the monarch to a position similar to that which existed pre-1707, where he or she was ministered by separate Ministries and Cabinets for each respective country. Thus, today, no Minister of the Crown in any Commonwealth realm can advise the monarch to exercise any powers pertaining to any of the other dominions.