2 intr. To exercise foresight in taking due measures in view of a possible event; to make provision or adequate preparation. Const. for, against.
c. To make it, or lay it down as, a provision or arrangement; to stipulate that. Cf. provided 5, providing pres. pple., provision 5.
That didn't sound 'legal' enough so I had a look at proviso.
[a. L. prōvīsō, abl. neut. sing. pa. pple. of prōvid-ēre to provide, as used in med.L. legal phrase prōvīsō quod ‘it being provided that’ (1350 in Du Cange).]
1 The L. ablative absolute = ‘it being provided’, used conjunctively. Obs. rare.
2 A clause inserted in a legal or formal document, making some condition, stipulation, exception, or limitation, or upon the observance of which the operation or validity of the instrument depends; a condition; hence, generally, a stipulation, provision.
"proviso". Oxford Dictionaries. Oxford University Press. http://www.oxforddictionaries.com/definition/english/proviso (accessed September 13, 2014).
A condition or qualification attached to an agreement or statement:
Origin - late Middle English: from the medieval Latin phrase proviso (quod) 'it being provided (that)', from Latin providere 'foresee, provide'.
OED1 is no longer copyrighted, you can find downloads and PDFs on the internet - there's a link somewhere on this very site to it. You'll find it more helpful for legal terms, that are antiquated in normal use, than most modern on-line dictionaries.
In this answer https://english.meta.stackexchange.com/a/2574/71783 [MετάEd https://english.meta.stackexchange.com/users/14073 ] there is a link to each volume of OED1 - almost every word you are looking for will be in there, with a complete list of all it's different (and obscure) meanings (except any that have been added in the last 100 years).