Extended Public Key

An extended public key, often written xpub and appearing under related prefixes depending on the address type, is the public half of a branch of a hierarchical deterministic wallet together with the chain code needed to derive further keys below it. From it, anyone can generate every receiving address that branch will ever produce, and therefore read the entire balance and transaction history of that account, without being able to spend anything. That asymmetry is the point: it is what lets watch-only wallets, accounting tools and merchant systems monitor addresses without holding spending authority. It is also the privacy consequence. Handing an extended public key to a service, or pasting it into a website, links every past and future address of that account to whoever holds it, permanently and with no way to revoke it. Sharing one is safe for the funds and expensive for privacy.