The amortized principal is the amount of money that has been repaid on a loan.
In a loan, the amortized principal refers to the amount of money that has actually been repaid out of the total debt. For example, in a loan of 100,000 euros with only 20,000 euros left to pay, the 80,000 euros already paid would be the amortized principal.
All the installments of a loan or a mortgage include amortized capital, which is the portion of the debt you repay, and interest, which is the interest agreed for the money that you have borrowed.