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]

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