Definition
An extended public key, almost always written as an xpub, is a BIP32 public key bundled with its chain code. From it, software can derive an unlimited sequence of child public keys and therefore every receive address in that branch of the wallet, all without any private key present. This is what powers watch-only wallets, hardware-wallet companion apps, and merchant point-of-sale systems that need fresh addresses but must never touch spending keys.
What the prefix tells you
The leading characters encode the script (address) type via version bytes registered in SLIP-0132. On Bitcoin mainnet: xpub maps to legacy P2PKH (BIP44, m/44'/0'), ypub to wrapped SegWit P2WPKH-in-P2SH (BIP49, m/49'/0'), and zpub to native SegWit bech32 P2WPKH (BIP84, m/84'/0'). Feeding a zpub into a wallet expecting an xpub produces the wrong addresses, a common cause of "missing funds" support tickets.
Privacy caveat
An xpub is read-only for spending but is highly revealing: anyone holding it can compute your entire address history and watch every payment you receive on that branch forever. Treat an xpub as sensitive. Share it only with services you trust, and prefer a dedicated account for any third party that needs one.
The xpub is a direct product of BIP32 HD wallets, and modern tooling increasingly wraps it inside an unambiguous output descriptor so the script type can never be misread.
In Simple Terms
An extended public key, almost always written as an xpub, is a BIP32 public key bundled with its chain code. From it, software can derive…