A loan default refers to the failure of a borrower to fulfil the financial obligations outlined in a loan agreement. When an individual or an entity borrows money from a lender, they are expected to repay the borrowed amount along with any interest or fees according to the agreed-upon terms and schedule. 

If the borrower fails to make timely payments or completely defaults on the loan, they are considered a loan defaulter. This can occur due to various reasons, such as financial hardship, unemployment, unexpected expenses, or mismanagement of funds. Loan defaulters can be individuals, businesses, or organisations that have borrowed money from banks, financial institutions, or other lending entities.