MSCD 12.148 and this May 2007 blog post consider the word indenture. Well, here’s another word for a particular kind of contract: deed.
Black’s Law Dictionary defines deed as “A written instrument by which land is conveyed” and “At common law, any written instrument that is signed, sealed, and delivered and that conveys some interest in property.” There are many varieties of deed—deed of trust, deed of adherence, and so on.
I have no objection to using the word deed for a particular kind of contract. But the idea of treating differently a contract under seal is, I suggest, counterproductive (see MSCD 4.33 and this January 2008 blog post). In a rational world, one would have no need for the second definition above.
But whether a contract is under seal or not continues to have significance in some U.S. states for purposes of the statute of limitations. And I welcome English readers to weigh in on the practical implications under English law of contracts under seal.