The person (or people) taking out the mortgage and legally responsible for paying it back. Can be one person or multiple co-borrowers.
The borrower is the person on the hook for repaying the mortgage. That's usually the buyer of the home, but it can be multiple people. If you and your spouse take out the loan together, you're both borrowers — both names go on the loan, both credit scores are evaluated, both incomes count toward qualifying, and both of you are equally responsible for paying it back.
It's worth noting: being a borrower isn't the same as being an owner. You can own the home (be on the deed) without being a borrower, and you can be a borrower without being on the deed. Most of the time these overlap, but in some situations (like a parent helping a child qualify), they don't.
Example
A married couple buying together typically goes on the loan as co-borrowers. Both their credit scores and incomes are considered. If one has weaker credit, sometimes the stronger spouse applies alone — but then only their income qualifies.
Also known as
applicant, mortgagor, primary borrower