The Collaborative International Dictionary of English v.0.48:

Imprison \Im*pris"on\, v. t. [imp. & p. p. Imprisoned; p. pr.
   & vb. n. Imprisoning.] [OE. enprisonen, OF. enprisoner, F.
   emprisonner; pref. en- (L. in) + F. & OF. prison. See
   Prison.]
   1. To put in prison or jail; To arrest and detain in custody;
      to confine.
      [1913 Webster]

            He imprisoned was in chains remediless. --Spenser.
      [1913 Webster]

   2. To limit, restrain, or confine in any way.
      [1913 Webster]

            Try to imprison the resistless wind.  --Dryden.

   Syn: To incarcerate; confine; immure.
        [1913 Webster]

WordNet (r) 3.0 (2006):

imprison
    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]
    2: confine as if in a prison; "His daughters are virtually
       imprisoned in their own house; he does not let them go out
       without a chaperone"

Moby Thesaurus II by Grady Ward, 1.0:

77 Moby Thesaurus words for "imprison":
   bastille, beleaguer, beset, besiege, blockade, bolt in, bound,
   box in, cage, cast in prison, chamber, check, circumscribe,
   clap in jail, clap up, close in, compass, confine, constrain,
   contain, coop, coop in, coop up, cordon, cordon off, corral, curb,
   detain, encircle, enclose, encompass, enshrine, fence in, gaol,
   hedge in, hem in, hold captive, hold in captivity, hold prisoner,
   house in, immure, impound, incarcerate, include, intern, jail, jug,
   kennel, leaguer, limit, lock in, lock up, mew, mew up, pen, pen in,
   pocket, prison, put away, quarantine, quod, rail in, release,
   remand, restrain, restrict, send down, shrine, shut in, shut up,
   stable, surround, throw into jail, wall in, wrap, yard, yard up