What is an Indenture?

Cornell law school says: 

1. Generally, any written agreement between two parties.
2. A real estate deed in which two parties agree to continuing obligations; for example, one party may agree to maintain the property and the other to make periodic payments.
3.  In finance, a written agreement that describes the borrowers' responsibility to the lenders in a bond or debenture issue and states the maturity date and the interest rate; also called a bond indenture.

https://www.law.cornell.edu/wex/indenture