Glossary / Addresses & keys
Unique address
- Definition
- A unique address appears in exactly one payment and is never handed out again, which is how every wallet built on BIP-32 behaves by default.
Specified in BIP-32.
The phrase carries two meanings and they are easy to mix up. Inside a wallet, a unique address is a hygiene rule: one address, one payment, then move on. On a block explorer's "unique addresses" chart it is a counting unit, and a poor proxy for how many people are using bitcoin.
How it works
A wallet produces unique addresses by walking an index, not by inventing new secrets. Under BIP-32 the receiving branch of an account has 2,147,483,648 children numbered from zero, and the receive screen shows the lowest one that has never appeared on chain. Nothing is generated in advance and nothing runs out.
Two settings keep that machinery honest. The gap limit, fixed at 20 in BIP-44, is how far a restored wallet scans past the last address with a payment before deciding there is nothing more to find. Bitcoin Core keeps a look-ahead pool of 1,000 keys for the same job. Both exist because a restored wallet cannot ask anybody which addresses it used; it has to derive candidates and search the chain for each one.
The gap limit is worth understanding before it bites. Generate 40 fresh addresses, use only the fortieth, then restore the seed into different software, and a wallet obeying a gap limit of 20 will scan the first twenty, find nothing, and report a balance of zero. The coins are safe and the fix is to raise the gap limit in the restoring wallet, but the ten minutes before you know that are unpleasant.
Where you see it
Unique addresses appear as a headline metric on explorer dashboards, where the number means considerably less than it looks like it means.
A daily unique address count tallies distinct addresses that touched the chain that day. One person making three payments from a modern wallet can create four or five of them once change is counted. An exchange serving two million customers may touch a few dozen, because it pools deposits and batches withdrawals into single transactions. The metric therefore tracks wallet design and exchange architecture at least as much as it tracks adoption, and it moves when a popular wallet changes its change policy rather than when anybody new arrives.
The wallet-side meaning has a newer form worth knowing. Silent payments, specified in BIP-352, let you publish one static string that never appears on chain at all: each sender uses it to compute a different one-time address, so you keep a reusable public identifier and every payment still lands somewhere unique. No consensus change is involved, only support in the sending and receiving software, which is why availability varies by wallet.
Invoices are the everyday case. A payment processor derives a unique address per order from an extended public key, which is why an invoice address is tied to a time window rather than to you personally. Pay late and the merchant's software may need to reconcile it by hand, not because the address stopped working, but because the order it belonged to has closed.