The Collaborative International Dictionary of English v.0.48:

Proviso \Pro*vi"so\, n.; pl. Provisos. [L., (it) being
   provided, abl. of provisus, p. p. of providere. See
   Provide, and cf. Purview.]
   An article or clause in any statute, agreement, contract,
   grant, or other writing, by which a condition is introduced,
   usually beginning with the word provided; a conditional
   stipulation that affects an agreement, contract, law, grant,
   or the like; as, the contract was impaired by its proviso.
   [1913 Webster]

         He doth deny his prisoners,
         But with proviso and exception.          --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

proviso
    n 1: a stipulated condition; "he accepted subject to one
         provision" [syn: provision, proviso]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "proviso":
   bill, boundary condition, calendar, catch, clause,
   companion bills amendment, condition, donnee, dragnet clause,
   enacting clause, escalator clause, escape clause, escape hatch,
   exception, fine print, given, grounds, hold-up bill, joker, kicker,
   limiting condition, motion, obligation, omnibus bill, parameter,
   prerequisite, privileged question, provision, provisions,
   qualification, question, requisite, reservation, rider,
   saving clause, sine qua non, small print, specification,
   stipulation, string, strings, term, terms, ultimatum, whereas



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

PROVISO. The name of a clause inserted in an act of the legislature, a deed,
a written agreement, or other instrument, which generally contains a
condition that a certain thing shall or shall not be done, in order that an
agreement contained in another clause shall take effect.
     2. It always implies a condition, unless subsequent words change it to
a covenant; but when a proviso contains the mutual words of the parties to a
deed, it amounts to a covenant. 2 Co. 72; Cro. Eliz. 242; Moore, 707 Com. on
Cov. 105; Lilly's Reg. h.t.; 1 Lev. 155.
     3. A proviso differs from an exception. 1 Barn. k Ald. 99. An exception
exempts, absolutely, from the operation of an engagement or an enactment; a
proviso defeats their operation, conditionally. An exception takes out of an
engagement or enactment, something which would otherwise be part of the
subject-matter of it; a proviso avoids them by way of defeasance or excuse.
8 Amer. Jurist, 242; Plowd. 361; Carter 99; 1 Saund. 234 a, note; Lilly's
Reg. h.t.; and the cases there cited. Vide, generally Amer. Jurist, No. 16,
art. 1; Bac. Ab. Conditions, A; Com. Dig. Condition, A 1, A 2; Darw. on
Stat. 660.