A proviso is something added to a document or agreement that details the terms. You might agree to buy a used car with a proviso that the fuzzy steering-wheel cover is included as part of the sale.
Proviso looks a lot like the words "provide" and "provision," and it goes back to the Latin verb providēre, "foresee, provide." Proviso often implies that there are conditions to getting something, as when you agree to take your younger brother to the movies with you, provided — with the proviso — that you get popcorn and an 82-ounce soft drink. Most parents would agree, with the proviso that the drink be no more than 20 ounces. Agreed?