Differentiate pledge, mortgage, hypothecation and lien?

Pledge-a serious or formal promise, especially one to give money or to be a friend, or something that you give as a sign that you will keep a promise

mortgage-an agreement which allows you to borrow money from a bank or similar organization, especially in order to buy a house or apartment, or the amount of money itself

Hypothecation–Hypothecation is the practice where a borrower pledges collateral to secure a debt

lien-an official order that allows someone to keep the property of a person who owes them money until it has been paid