An invoice is a detailed written record that shows exactly what someone owes for goods or services they've received. When a business delivers something, whether it's repairing a car, catering a party, or supplying materials for a construction project, they send an invoice listing everything provided, the cost of each item, and the total amount due.

Think of an invoice as more formal and detailed than a simple receipt. A receipt proves you already paid for something, but an invoice arrives first, showing what you need to pay and when payment is due. If your school orders new computers, the supplier sends an invoice stating “50 laptops at $800 each, total: $40,000, payment due within 30 days.”

Invoices protect both sides of a transaction. The buyer gets a clear record of what they're paying for, while the seller has proof of what they delivered and what's owed. Businesses typically number their invoices sequentially and keep copies for their records.

In everyday conversation, you might hear someone say they need to invoice a client, using the word as a verb meaning to send an invoice or bill someone officially for work completed.