n. — Politics, Administration
the federal prime minister or a provincial/territorial premier.
The term first minister refers to either the prime minister or a provincial premier. The earliest Canadian attestations of the term appear to be used in the full title of "First Minister of the Crown" (see the 1847 and 1973 quotations). Appearing almost 100 years after the earliest found UK source in 1741 (see OED-3, s.v. "first" (4b), the term is Canadian only in a technical sense, i.e. the First Minister of a Canadian executive vs. another country's. The term is not very frequent in Canada (see Chart 1), which is in marked contrast to the plural form First Ministers. The most prevalent Canadian form is premier.
See also COD-2, s.v. "first minister" (1) and (2), which are marked as "Cdn".
See also: premier (def. 1) First Ministers
- 1847  As the First Minister of the Crown, Mr Baldwin would be safe for the Sovereign, and the faithful guardian of the rights of the people. 
- 1873  It was a well known principle that when the First Minister of the Crown resigned, or when by death his offices[?] was vacated, the Ministry was dissolved. 
- 1899  "The actual work which such a Premier as Mackenzie, Macdonald or Sir Wilfrid Laurier does, quite apart from the anxieties of the position, is intellectually Herculean, and such as is not bought in commerce and industry except for vastly higher salaries than we pay to our First Ministers. It is too much, as The Gazette points out, to ask that in addition to the burden of care and responsibility for great public affairs a Premier should have to struggle, perhaps vainly, with insufficient means to bear the extra private burdens entailed by his public position." 
- 1912  The Right Honorable Robert Laird Borden is Canada's Premier to-day. The country has reason to congratulate itself on the quality of the eight men who have served her interests as First Ministers since 1867, and Mr. Borden enters upon his responsible task with a well-won character for integrity and high ideals of duty. Never has a Canadian Premier had such a chance to do good work for his country; never have the gods been more generous in their gifts than to this Britain of the West. 
- 1922  "It does get a bit tiresome when the First Minister in the Province of Ontario acts like a 10-year-old youngster in this House every day." 
- 1947  He is one of the few men in the history of the province to become first minister of the Crown. 
- 1999  However, it was a speech by Canada's newest first minister, Paul Okalik, that drew the warmest reaction from the audience. 
- 2008  The three opposition leaders wrote directly to MichaĆ«lle Jean, in effect breaching the convention that the Governor-General takes advice only from her first minister. (Notably, David Peterson did not follow this path to the premier's office.) 
- 2014  The past 30 years Canadians have been told first ministers, premiers and prime ministers alike, reign in splendid isolation. We're told, by pundits, academics and even parliamentarians, that first ministers decide everything and exert an Orwellian level of control. [...] First ministers don't reign alone. Politics is a team endeavour and a successful first minister must balance the personal dynamics of caucus and cabinet to stay in office. 
Images:
Chart 1: Internet Domain Search, 3 Jun. 2014