In one sentence
The table setting out how each payment splits between principal and interest over the loan term.
The longer version
Amortization determines how fast the balance pays down. A 25-year amortization on a 10-year loan means a substantial balloon at maturity; full amortization eliminates it. The schedule drives payment size and payoff math.
Servicers board the schedule from the loan documents, and errors here create payment disputes, which is why boarding verifies against source documents.
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