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]