The Collaborative International Dictionary of English v.0.48:

Restrain \Re*strain"\, v. t. [imp. & p. p. Restrained; p. pr.
   & vb. n. Restraining.] [OE. restreinen, F. restreindre, fr.
   L. restringere, restrictum; pref. re- re- + stringere to
   draw, bind, or press together. See Strain, v. t., and cf.
   Restrict.]
   1. To draw back again; to hold back from acting, proceeding,
      or advancing, either by physical or moral force, or by any
      interposing obstacle; to repress or suppress; to keep
      down; to curb.
      [1913 Webster]

            Restrain in me the cursed thoughts that nature
            Gives way to in repose!               --Shak.
      [1913 Webster]

   2. To draw back toghtly, as a rein. [Obs.] --Shak.
      [1913 Webster]

   3. To hinder from unlimited enjoiment; to abridge.
      [1913 Webster]

            Though they two were committed, at least restrained
            of their liberty.                     --Clarendon.
      [1913 Webster]

   4. To limit; to confine; to restrict. --Trench.
      [1913 Webster]

            Not only a metaphysical or natural, but a moral,
            universality also is to be restrained by a part of
            the predicate.                        --I. Watts.
      [1913 Webster]

   5. To withhold; to forbear.
      [1913 Webster]

            Thou restrained prayer before God.    --Job. xv. 4.
      [1913 Webster]

   Syn: To check; hinder; stop; withhold; repress; curb;
        suppress; coerce; restrict; limit; confine.
        [1913 Webster]

WordNet (r) 3.0 (2006):

restrain
    v 1: keep under control; keep in check; "suppress a smile";
         "Keep your temper"; "keep your cool" [syn: restrain,
         keep, keep back, hold back]
    2: place limits on (extent or access); "restrict the use of this
       parking lot"; "limit the time you can spend with your
       friends" [syn: restrict, restrain, trammel, limit,
       bound, confine, throttle]
    3: to close within bounds, limit or hold back from movement;
       "This holds the local until the express passengers change
       trains"; "About a dozen animals were held inside the
       stockade"; "The illegal immigrants were held at a detention
       center"; "The terrorists held the journalists for ransom"
       [syn: restrain, confine, hold]
    4: hold back [syn: restrain, encumber, cumber,
       constrain]
    5: to compel or deter by or as if by threats [syn: intimidate,
       restrain]

Moby Thesaurus II by Grady Ward, 1.0:

225 Moby Thesaurus words for "restrain":
   abate, adjust to, allay, alleviate, alter, anchor, arrest, assuage,
   attemper, bank the fire, bar, bind, bit, block, blunt, bottle up,
   bound, box in, box up, bridle, cage, cause, cause to, chain,
   chasten, check, circumscribe, cloister, coarct, collect, compel,
   condition, confine, constrain, contain, control, cool, cool off,
   coop, coop in, coop up, copyright, cork up, countercheck, crib,
   crimp, curb, curtail, dam up, damp, dampen, de-emphasize, deaden,
   debar, decelerate, delay, detain, diminish, discipline, dompt,
   downplay, draw the line, drive, dull, encage, enchain, enclose,
   enforce, enjoin, entrammel, extenuate, fasten, fence in, fetter,
   forbear, force, gag, govern, guard, gyve, hamper, handcuff,
   handicap, have, hedge, hedge about, hem in, hinder, hobble,
   hog-tie, hold, hold at bay, hold back, hold down, hold fast,
   hold in, hold in check, hold in custody, hold in leash,
   hold in restraint, hold up, hopple, immure, impede, impel, impound,
   imprison, incarcerate, inhibit, intercept, interfere,
   interfere with, intermeddle, interrupt, intervene, keep, keep back,
   keep from, keep in, keep in check, keep in custody,
   keep in detention, keep under control, keep within bounds, lash,
   lay, lay under restraint, leash, leaven, lenify, lessen, lighten,
   limit, lock up, make, make fast, manacle, meddle, mew, mew up,
   mitigate, moderate, modify, modulate, moor, muzzle, narrow,
   obstruct, obtund, oppose, palliate, patent, peg down, pen, pen up,
   picket, pin down, pinion, play down, pound, prevent, prohibit,
   pull, pull in, purify, put in irons, qualify, rail in, reduce,
   reduce the temperature, refrain, register, regulate, regulate by,
   rein, rein in, repress, resist, restrict, retard, retrench, rope,
   scant, scotch, seal up, season, secure, set back, set conditions,
   set limits, shackle, shut in, shut up, simmer down, simplify,
   slacken, slow down, smother, snub, sober, sober down, soften,
   specialize, stifle, stint, stop, straiten, straitjacket, strap,
   subdue, suppress, tame, temper, tether, tie, tie down, tie up,
   tone down, trammel, tune down, underplay, use force upon, wall in,
   weaken, withhold



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

RESTRAINT. Something which prevents us from doing what we would desire to
do.
     2. Restraint is lawful and unlawful. It is lawful when its object is to
prevent the violation of the law, or the rights of others. It is unlawful
when it is used to prevent others from doing a lawful act; for example, when
one binds himself not to trade generally; but an agreement not to trade in a
particular place is lawful. A legacy given in restraint of marriage, or on
condition that the legatee shall not marry, is good, and the condition alone
is void. The Roman civil law agrees with ours in this respect; a legacy
given on condition that the legatee shall not marry is void. Clef des Lois
Rom. mot Passion. See Condition; Limitation.