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.
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.
A proviso differs from an exception. 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.