Parol evidence
An old fashioned idea that once a contract is reduced to writing, any other evidence of the parties’ agreement or intentions must be ignored: in many ways a companion piece to the equally fusty “no oral modification” rule, which in the hands of well meaning commercial judges is enjoying a bit of an indian summer. In practice there are exceptions, and you will usually need at least an entire agreement clause, unless your contract is of the sort that is required by law to be in writing.