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A clause in a legal instrument, such as a contract, deed, or statute, requiring that something must occur or not occur before another part of the agreement, or the entire legal instrument itself, can become valid. The word comes from the Mediaeval Latin term proviso quod, meaning “provided that.” (e.g., Provided that X occurs, Y can take effect.)

When a proviso is not fulfilled in a contract, it usually impacts the validity of the entire legal instrument. In a statute, however, the fulfillment or nonfulfillment of the condition a proviso usually creates an exception from a statutory requirement or qualifies the statute.

The term proviso also appears in a number of philosophical and political contexts. For instance, the Lockean proviso is a condition to John Locke’s labor theory of property, stating that even though each individual who works to turn natural resources into property has a right to said property, he or she can only have a right to such property “at least where there is enough, and as good, left in common for others.”
In American history, the 1846 Wilmot Proviso was a failed attempt by Congress to ban slavery in the new territories ceded to the US after the Mexican–American War—and a catalyst to the American Civil War. The proviso states: “Provided that, as an express and fundamental condition to the acquisition of any territory from the Republic of Mexico by the United States, by virtue of any treaty which may be negotiated between them, and to the use by the Executive of the moneys herein appropriate, neither slavery nor involuntary servitude shall ever exist in any part of said territory, except for crime, whereof the party shall first be duly convicted.”

[Last updated in August of 2020 by the Wex Definitions Team]