💡 The Plain-English Definition
A Lightning invoice is a payment request — a short piece of data, often displayed as a QR code, that tells the sender exactly how much to pay, where to send it, and when the request expires. It’s the standard way to receive a Lightning payment.
🤔 But Why Though?
On Bitcoin’s base layer, you can share the same address repeatedly to receive multiple payments — though this has privacy downsides. Lightning works differently. Each payment is routed using a specific payment hash (a unique cryptographic identifier generated for that specific transaction), and once used, that hash is spent — it can’t be reused for another payment. This means Lightning payments require a fresh invoice for each transaction.
The BOLT11 standard (Basis of Lightning Technology 11 — the specification that defines Lightning invoice format) encodes several pieces of information into a compact string beginning with “lnbc” (for Bitcoin mainnet). The payment amount, the destination node’s public key, the payment hash, the invoice expiry time (typically ten minutes to one hour — after which it’s invalid), and an optional description. When you scan a Lightning QR code in a wallet or coffee shop, your wallet reads this encoded data, finds a route to the destination, and sends the exact amount to the exact payment hash. The difference from a Bitcoin address is meaningful: a Bitcoin address is a destination that can receive any amount at any time, indefinitely. A Lightning invoice is a single-use payment request for a specific amount with a defined expiry — closer to an invoice from a shop than an address on a letter.
🌍 The Real-World Analogy
A Lightning invoice is like a parking ticket payment code — a specific reference number tied to a specific amount owed, with an expiry time, issued for exactly one payment. You can’t use the same parking code to pay two different fines. You can’t use a code that expired yesterday. The code encodes everything the payment system needs — who issued it, how much, when it expires — in a compact scannable format. A Lightning invoice does exactly the same for Bitcoin payments.
⚡ So What?
Invoices are the normal user interface for receiving Lightning payments — you’ll generate them from your wallet whenever someone wants to pay you. The practical things to know: invoices expire, so don’t generate one until you’re ready to share it. The amount is fixed — you can’t pay more or less than the invoice specifies (though zero-amount invoices exist for cases where the sender sets the amount). If an invoice expires before payment, simply generate a new one.