The Collaborative International Dictionary of English v.0.48:

Gaol \Gaol\ (j[=a]l), n. [See Jail.]
   A place of confinement, especially for minor offenses or
   provisional imprisonment; a jail. [Preferably, and in the
   United States usually, written jail.]
   [1913 Webster]

   Commission of general gaol delivery, an authority conferred
      upon judges and others included in it, for trying and
      delivering every prisoner in jail when the judges, upon
      their circuit, arrive at the place for holding court, and
      for discharging any whom the grand jury fail to indict.
      [Eng.]

   Gaol delivery. (Law) See Jail delivery, under Jail.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Jail \Jail\ (j[=a]l), n. [OE. jaile, gail, gayhol, OF. gaole,
   gaiole, jaiole, F. ge[^o]le, LL. gabiola, dim. of gabia cage,
   for L. cavea cavity, cage. See Cage.]
   A kind of prison; a building for the confinement of persons
   held in lawful custody, especially for minor offenses or with
   reference to some future judicial proceeding. [Written also
   gaol.]
   [1913 Webster]

         This jail I count the house of liberty.  --Milton.
   [1913 Webster]

   Jail delivery, the release of prisoners from jail, either
      legally or by violence.

   Jail delivery commission. See under Gaol.

   Jail fever (Med.), typhus fever, or a disease resembling
      it, generated in jails and other places crowded with
      people; -- called also hospital fever, and ship fever.


   Jail liberties, or Jail limits, a space or district
      around a jail within which an imprisoned debtor was, on
      certain conditions, allowed to go at large. --Abbott.

   Jail lock, a peculiar form of padlock; -- called also
      Scandinavian lock.
      [1913 Webster]

WordNet (r) 3.0 (2006):

gaol
    n 1: a correctional institution used to detain persons who are
         in the lawful custody of the government (either accused
         persons awaiting trial or convicted persons serving a
         sentence) [syn: jail, jailhouse, gaol, clink,
         slammer, poky, pokey]
    v 1: lock up or confine, in or as in a jail; "The suspects were
         imprisoned without trial"; "the murderer was incarcerated
         for the rest of his life" [syn: imprison, incarcerate,
         lag, immure, put behind bars, jail, jug, gaol,
         put away, remand]

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "gaol":
   POW camp, bastille, beleaguer, beset, besiege, black hole,
   blockade, bolt in, borstal, borstal institution, bound, box in,
   bridewell, brig, cage, cast in prison, cell, chamber, clap in jail,
   clap up, close in, compass, concentration camp, condemned cell,
   contain, coop, coop in, coop up, cordon, cordon off, corral,
   death cell, death house, death row, detention camp, encircle,
   enclose, encompass, enshrine, federal prison, fence in,
   forced-labor camp, guardhouse, hedge in, hem in, hold captive,
   hold in captivity, hold prisoner, house in, house of correction,
   house of detention, immure, impound, imprison, incarcerate,
   include, industrial school, intern, internment camp, jail,
   jailhouse, jug, keep, kennel, labor camp, leaguer, lock in,
   lock up, lockup, maximum-security prison, mew, mew up,
   minimum-security prison, oubliette, pen, pen in, penal colony,
   penal institution, penal settlement, penitentiary, pocket, prison,
   prison camp, prisonhouse, quarantine, rail in, reform school,
   reformatory, shrine, shut in, shut up, sponging house, stable,
   state prison, stockade, surround, the hole, throw into jail,
   tollbooth, training school, wall in, wrap, yard, yard up



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

GAOL. A prison or building designated by law or used by the sheriff, for the
confinement or detention of those, whose persons are judicially ordered to
be kept in custody., This word, sometimes written jail, is said to be
derived from the Spanish jaula, a cage, (derived from caula,) in French
geole, gaol. 1 Mann. & Gran. 222, note a. Vide 6 John. R. 22; 14 Vin. Ab. 9;
Bac. Ab. h. t.; Dane's Ab. Index, h. t.; 4 Com. Dig. 619; and the articles
Gaoler; Prison; Prisoner.