Public vs. Private Blockchain

A public blockchain is one whose transaction data and ledger history anyone can read, verify, and independently audit, with no central gatekeeper controlling who can view that data — Bitcoin and Ethereum are both public blockchains in this sense. A private blockchain, by contrast, restricts data access to a specific, authorized group of participants, such as the employees or partners of a single organization running it.

This public-versus-private distinction concerns data visibility specifically, and is related to but distinct from the permissioned-versus-permissionless (see Kaavon's "Permissioned vs. Permissionless Blockchain" glossary definition) distinction, which concerns who can participate in validating transactions — a blockchain can, in principle, mix these properties in different combinations, such as being privately readable but still permissionlessly validated, though in practice private blockchains are almost always also permissioned, since an organization restricting data visibility typically also wants to control who can participate in validation.