A zap is a tiny Bitcoin payment, announced as a Nostr event. The money moves over Lightning, the receipt gets posted to relays, and everyone sees the zap appear next to the post it was sent to. Think of it as a like that carries actual value.
How it works
Behind the scenes a zap is a two-step dance between a Lightning wallet and a Nostr client. The sender's client asks the recipient's Lightning address for an invoice, with a Nostr event attached describing what is being tipped. The wallet generates the invoice; the sender pays it; the wallet publishes a kind:9735 "zap receipt" event signed by the wallet service, referencing the original zap request. Any client following the recipient now sees the tip.
The user does not see any of this. From the feed, zapping looks like tapping a lightning-bolt icon, picking an amount, and watching the counter go up.
Why people care
Three reasons zaps are different from plain Lightning tips outside Nostr.
First, they are public by default. A zap receipt is a regular Nostr event, so it shows up on feeds, leaderboards, and search results. Writers get direct visible feedback on specific posts; podcasts and streams track zap amounts in real time.
Second, the cost is honest. No platform takes a cut, no processor charges a fee, the sats go straight from sender to receiver's wallet. A 21-sat zap is about 1 cent and arrives in seconds.
Third, zaps are composable. Because the receipt is a Nostr event like any other, other clients and services can build on top: aggregate top-zapped posts of the week, split payments between collaborators, trigger notifications. It is a tipping protocol, not a tipping feature.
What can go wrong
Zaps need Lightning on both ends. If either side has not set up a wallet or a zap-capable Lightning address, the interaction fails. Some wallets say "zap enabled" but skip publishing the receipt, in which case the money moves but no one sees it. This is a common source of confusion for new users.
The receipt is signed by the wallet service, not by the sender. This means someone could technically forge a receipt with a bogus wallet. In practice, most clients filter out receipts signed by wallets they do not trust, but a raw Nostr event stream will show them all.