A blockchain is a distributed ledger that records transactions in time-ordered blocks, each cryptographically linked to the previous one. No central operator controls it; instead, a network of independent nodes agrees on the state via a consensus mechanism (proof-of-work or proof-of-stake).
The design solves one specific problem: how to transfer value over the internet without a trusted third party. Before Bitcoin's 2009 launch, every digital-money proposal required a central operator who could be compelled, censored, or corrupted. A blockchain replaces that operator with math.
In practice, modern crypto projects extend the design beyond money: smart-contract chains (Ethereum, Solana, Aptos) execute arbitrary programs on-chain; app-specific chains (Cosmos, Polkadot) host sovereign applications; rollups (Arbitrum, Optimism) batch transactions off-chain while inheriting a base chain's security.
What blockchains are not: databases. Every participant stores the full history, which makes them slow and expensive for high-throughput use cases where trust is already solved through other means.