A document requesting payment for goods or services delivered.
For example, after completing a $12,000 commercial painting job, the contractor sends the client an invoice dated June 15th with payment due in 30 days — this document triggers the accounts receivable entry in the contractor's books and the accounts payable entry in the client's books.
Why it matters: It triggers the accounts receivable cycle; clear, prompt invoicing is the first defense against cash flow delays.