Debt is when you owe money to someone.
Yes, or it could be a moral debt as well couldn't it?
yes it could, the borrower can be a state, country, government, company or person
Are we talking bonds for stuff like building roads and the like?
yes that is exactly right, the term debt was first used in the 13 century
Wasn't there a law that Christians couldn't loan money back then?
not that i know of but could be a debt of gratitude.