Relating to a loan, this is the Certificate or voucher acknowledging the existence of a debt. Often listed on the Balance Sheet, it shows the company's obligation to repay such a loan or 'Debenture' the rate of interest due, and the timing of its repayment.
Just a fancy type of loan - where companies write documentation to cover the basis of the advance and the interest and repayment terms. Don't get intimidated by a big name for something so simple. (Why don't you look at the serious definition... go,on)