n. & adj. — Law, Administration
a prison.
Originally, a penitentiary was a place one went to "atone for ecclesiastical offences" (see OED-3, s.v. "penitentiary" (n2.1)). Some of the earliest Canadian quotations adhere to this meaning (see, e.g. the 1790 quotation). Currently, a penitentiary in Canada is a federal prison. The differences of usage in the US are minimal and one must go into technicalities of the legal systems and their terms to see a Canadian dimension in the present meaning. COD-2's definition is for a federal prison for persons serving sentences of two or more years. This is technically correct today, but not matched by the use of the average Canadian English speaker. In the US, penitentiary may refer to a state or a federal prison (see DA, s.v. "penitentiary" and W-3, s.v. "penitentiary" (4)), or to a prison generally (see AHD-5, s.v. "penitentiary" (1) which is described as a prison for persons convicted of major crimes; see DAE, s.v. "penitentiary" (1.b)).
The term penitentiary is most prevalent in several countries other than Canada, including the US (see Chart 1). The present definition is recent; see, for instance, the 1849 quotation, which speaks of provincial penitentiaries, a category not possible according to the current official use of the term.
See, in contrast, COD-2, s.v. "penitentiary" (1.a), which marks the meaning "Cdn", for which we find little evidence.
- 1790  The solitude, the temperance, and the industry proposed in the penitentiary plan as laid down by Sir Jerome Fitzpatrick, adopted in England -- and now patronized by Government in this country -- most certainly promises an effectual remedy for this alarming progress of vice. 
- 1847  Western District Assizes.-- In our last week's impression we were enabled to give only a short account of the Assizes, inasmuch as the Court was still sitting when we went to press. We now return to the subject, in order to acquaint our readers with the sentences passed upon the criminals convicted. The prisoners White and Rily were sentenced to three years' imprisonment in the Penitentiary. 
- 1849  STATEMENT OF THE ACCOUNTS AND AFFAIRS OF THE PROVINCIAL PENITENTIARY FOR THE YEAR 1848; LAID BEFORE THE HONOURABLE THE LEGISLATIVE ASSEMBLY, ON THE 4TH OF MAY, 1849. 
- 1873  Some sixty or seventy convicts will shortly be removed from the Kingston Penitentiary to the new Penitentiary at St. Vincent De Paul. 
- 1900  Convicts Dufresne and Godwin have arrived at the penitentiary here to finish their terms in the insane department of that institution. They have to serve ten and three years respectively. 
- 1929  The year 1929 has been probably the most successful year for the department in cleaning up crime and in the number of persons apprehended, convicted and sentenced to terms in the penitentiary. 
- 1951  The reasons why most of the convicts the author knew in the penitentiary were not penitent were, he said, the capriciousness of the written law and its interpretation, the corruption of law enforcement and the respectability of white-collar criminals. 
- 1977  The Canadian Penitentiary Service has stopped moving prisoners on commercial flights and will use military aircraft from now on for transfers, a spokesman said yesterday.
The move follows an observation in a special Commons subcommittee report last June that indicated members were concerned about the movement of dangerous prisoners on commercial aircraft. 
- 1991  In a similar ruling in 1990, the Federal Court of Canada ruled that Timothey Veysey, a prisoner in a federal penitentiary, should be allowed family visits with his homosexual partner. 
- 2013  A few years ago I had the great fortune of visiting someone in jail. Fortune? Yes -- I was dreadfully glad I wasn't on the other side of the glass!
It was summertime, but the prisoners were a sickly yellow. No one in that prison had been outside for six months. The penitentiary was located near a river and there had been too many escapes, so the fresh air privilege was gone. 
Images:
Chart 1: Internet Domain Search, 20 Aug. 2012