What is Proviso?

Legal Definition
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; Dwar. on Stat. 660.
-- Bouviers Law Dictionary
Legal Definition
A condition or provlaion which is inserted in a deed, lease, mortgage or contract, and on the performance or nonperformance of which the validity of the deed, etc., frequently depends; it usually begins with the word "provided." A proviso in deeds or laws is a limitation or exception to a grant made or authority conferred, the effect of which is to declare that the one shall not operate, or the other be exercised, unless in the case provided. Voorhees v. Bank of United States, 10 Pet. 449, 9 L. Ed. 490. The word "proviso" is generally taken for a condition, but it differs from it in several respects ; for a condition is usually created by the grantor or lessor, but a proviso by the grantee or lessee. Jacob. A proviso differs from an exception. 1 Barn. & Aid. 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 Am. Jur. 242. A clause or part of a clause in a statute, the office of which is either to except something from the enacting clause, or to qualify or restrain its generality, or to exclude some possible ground of misinterpretation of its extent Minis v. In S., 15 Pet. 445, 10 In Ed. 791; In re Matthews (D. C.) 109 Fed. 614; Carroll v. State, 58 Ala. 396; Waffle v. Goble, 53 Barb. (N. Y.) 522. Proviso est providere praesentia et futura, non praeterita. Coke, 72. A proviso is to provide for the present or future, not the past.
-- Black's Law Dictionary
Legal Definition
A word suggesting prevision. “Look out for it. See that the general words of the enacting clause shall not have a particular effect.” See 59 Cal. 483. See, also, Trial by proviso.
-- Ballentine's Law Dictionary