The Collaborative International Dictionary of English v.0.48:

Penitentiary \Pen`i*ten"tia*ry\
   (p[e^]n`[i^]*t[e^]n"sh[.a]*r[y^]), a. [Cf. F.
   p['e]nitentiaire.]
   1. Relating to penance, or to the rules and measures of
      penance. "A penitentiary tax." --Abp. Bramhall.
      [1913 Webster]

   2. Expressive of penitence; as, a penitentiary letter.
      [1913 Webster]

   3. Used for punishment, discipline, and reformation.
      "Penitentiary houses." --Blackstone.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Penitentiary \Pen`i*ten"tia*ry\, n.; pl. Penitentiaries. [Cf.
   F. p['e]nitencier. See Penitent.]
   1. One who prescribes the rules and measures of penance.
      [Obs.] --Bacon.
      [1913 Webster]

   2. One who does penance. [Obs.] --Hammond.
      [1913 Webster]

   3. A small building in a monastery where penitents confessed.
      --Shpiley.
      [1913 Webster]

   4. That part of a church to which penitents were admitted.
      --Shipley.
      [1913 Webster]

   5. (R. C. Ch.)
      (a) An office of the papal court which examines cases of
          conscience, confession, absolution from vows, etc.,
          and delivers decisions, dispensations, etc. Its chief
          is a cardinal, called the Grand Penitentiary,
          appointed by the pope.
      (b) An officer in some dioceses since A. D. 1215, vested
          with power from the bishop to absolve in cases
          reserved to him.
          [1913 Webster]

   6. A house of correction, in which offenders are confined for
      punishment, discipline, and reformation, and in which they
      are generally compelled to labor; a prison; a jail.
      Colloquially often shortened to pen.
      [1913 Webster]

WordNet (r) 3.0 (2006):

penitentiary
    adj 1: used for punishment or reform of criminals or wrongdoers;
           "penitentiary institutions"
    2: showing or constituting penance; "penitential tears"; "wrote
       a penitential letter apologizing for her hasty words" [syn:
       penitential, penitentiary]
    n 1: a correctional institution for those convicted of major
         crimes [syn: penitentiary, pen]

Moby Thesaurus II by Grady Ward, 1.0:

99 Moby Thesaurus words for "penitentiary":
   Grand Penitentiary, Holy Father, POW camp, abuna, antipope,
   archbishop, archdeacon, archpriest, bail, bastille, bishop,
   bishop coadjutor, black hole, borstal, borstal institution,
   bridewell, brig, can, canon, cardinal, cardinal bishop,
   cardinal deacon, cardinal priest, cassock, cell, chaplain,
   coadjutor, concentration camp, condemned cell, confessor, cooler,
   coop, curate, cure, dean, death cell, death house, death row,
   detention camp, diocesan, ecclesiarch, exarch, father,
   father confessor, father in Christ, federal prison,
   forced-labor camp, gallach, gaol, guardhouse, hierarch,
   high priest, hoosegow, house of correction, house of detention,
   industrial school, internment camp, jail, jailhouse, keep,
   labor camp, lockup, maximum-security prison, metropolitan,
   minimum-security prison, oubliette, padre, papa, parish priest,
   patriarch, pen, penal colony, penal institution, penal settlement,
   pontiff, pope, prebendary, prelate, presbyter, priest, primate,
   prison, prison camp, prisonhouse, rector, reform school,
   reformatory, rural dean, spiritual director, spiritual father,
   sponging house, state prison, stockade, subdean, suffragan,
   the hole, tollbooth, training school, vicar



Bouvier's Law Dictionary, Revised 6th Ed (1856):

PENITENTIARY. A prison for the punishment of convicts.
     2. There are two systems of penitentiaries in the United States, each
of which is claimed to be the best by its partisans: the Pennsylvania system
and the New York system. By the former, convicts are lodged in separate,
well lighted, and well ventilated cells, where they are required to work,
during stated hours. During the whole time of their confinement, they are
never permitted to see or speak with each other. Their usual employments are
shoemaking, weaving, winding yarn, picking wool, and such like business. The
only punishments to which convicts are subject, are the privation of food
for short periods, and confinement without labor in dark, but well aired
cells; this discipline has been found sufficient to keep perfect order; the
whip and all other corporal punishments are prohibited. The advantages of
the plan are numerous. Men cannot long remain in solitude without labor
convicts, when deprived of it, ask it as a favor, and in order to retain it,
use, generally, their best exertions to do their work well; being entirely
secluded, they are of course unknown to their fellow prisoners, and can form
no combination to escape while in prison, or associations to prey upon
society when they are out; being treated with kindness, and afforded books
for their instruction and amusement, they become satisfied that society does
not make war upon them, and, more disposed to return to it, which they are
not prevented from doing by the exposure of their fellow prisoners, when in
a strange place; the labor of the convicts tends greatly to defray the
expenses of the prison. The disadvantages which were anticipated have been
found, to be groundless.; Among these were, that the prisoners would be
unhealthy; experience has proved the contrary; that they would become
insane, this has also been found to be otherwise; that solitude is
incompatible with the performance of business; that obedience to the
discipline of the prison could not be enforced. These and all other
objections to this system are, by its friends, believed to be without force.
     3. The New York system, adopted at Auburn, which was probably copied
from the penitentiary at Ghent, in the Netherlands, called La Maison de
Force, is founded on the system of isolation and separation, as well as that
of Pennsylvania, but with this difference, that in the former the prisoners
are confined to their separate cells during the night only; during the
working hours in the day time they labor together in work shops appropriated
to their use. They cat their meals together, but in such a manner as not to
be able to speak with each other. Silence is also imposed upon them at their
labor. They perform the labor of carpenters, blacksmiths, weavers,
shoemakers, tailors, coopers, gardeners, wood sawyers, &c. The discipline of
the prison is enforced by stripes, inflicted by the assistant keepers, on
the backs of the prisoners, though this punishment is rarely exercised. The
advantages of this plan are, that the convicts are in solitary confinement
during the night; that their labor, by being joint, is more productive;
that, inasmuch as a clergyman is employed to preach to the prisoners, the
system affords an, opportunity for mental and moral improvements. Among the
objections made to it are, that the prisoners have opportunities of
communicating with each other, and of forming plans of escape, and when they
are out of prison, of associating together in consequence of their previous
acquaintance, to the detriment of those who wish to return to virtue, and to
the danger of the public; that the discipline is degrading, and that it
engenders bitter resentment in the mind of the convict. Vide, generally, on
the subject of penitentiaries, Report of the Commissioners (Messrs. King,
Shaler, and Wharton,) on the Penal Code of Pennsylvania; De Beaumont and De
Toqueville, on the Penitentiary System of the United States; Mease on the
Penitentiary System of Pennsylvania; Carey on ditto; Reports of the Boston
Prison Discipline Society; Livingston's excellent Introductory Report to the
Code of Reform and Prison Discipline, prepared for the state of Louisiana;
Encycl. Americ. art. Prison Discipline; De. I'Etat Actuel des Prisons en
France, par L. M. More au Christophe; Dalloz, Dict. mot Peine, Sec. 1, n. 3,
and Supplem. mots Prisons et Bagnes.