"Indenture" literally meant a broken edge of a document that had been torn in two. Because land ownership was so important, it was essentially to keep clean title, and part of that was to prevent fraudulent documents. When a deed was executed, two copies would be drawn up on the same document, and it would then be torn into two copies of the document. The rough edge where the tear occured could be compared to make sure both copies were authentic. That edge was the "indenture," and I guess at some point they started referring to the document itself as an indenture.