A keysend payment in the Lightning Network is a type of transaction that allows for spontaneous payments without needing an invoice from the recipient. In traditional HTLC (Hash Time-Locked Contract) transactions, the receiver generates an invoice with a payment hash, and upon payment by the sender, the receiver reveals a secret preimage to claim the funds. With keysend, this process is inverted—the sender generates the preimage and includes it within the onion payload sent to the receiver. The receiver then uses this preimage, which must match the payment hash of the HTLC, to settle the payment.

Keysend payments use a custom record type (5482373484) to encode a 32-byte preimage value. This means that payments can be made directly to a recipient identified by their public key without the need for an invoice. This feature enables new use cases such as tipping or donations where obtaining an invoice might not be practical or possible.

However, there are drawbacks to keysend payments. For example, the payee cannot specify their preferred minimum CLTV expiry, which may lead to security issues or payment failures. Moreover, since no invoice is issued, there is a loss of proof-of-payment capability, which is typically provided by the invoice signature and preimage in regular Lightning payments.

Furthermore, keysend is expected to be deprecated in favor of newer spontaneous payment solutions like Offers or AMP (Atomic Multi-Path Payments) because these provide better security features and user experience.

In summary, keysend enables spontaneous, invoice-less payments on the Lightning Network, but it comes with certain limitations and is likely to be replaced by more advanced protocols in the future.