In one sentence
The document by which a lender formally commits to fund a loan, stating terms, conditions, expiration, and any deposit.
The longer version
The commitment letter is the borrower's assurance the money is real and the lender's statement of what must be true before it moves. It carries an expiration that pressures the closing calendar.
Deposits and rate-lock mechanics often attach here, which is why the letter is negotiated with care even though the loan documents supersede it at closing.
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