Blockchain is the data structure that made cryptocurrency possible. At the most literal level, it is a list of blocks where each block contains a hash of the previous block, so that any attempt to change history further back would break every subsequent block. This linkage is what makes the history of a blockchain tamper-evident: you cannot rewrite a transaction from three years ago without also rewriting every block that came after it and producing new valid proofs for each one. For any sufficiently large network, that is computationally hopeless.
The word “blockchain” is now used so loosely that it no longer tells you much on its own. Sometimes it means the data structure. Sometimes it means the software that maintains the data structure. Sometimes it means the network of nodes running that software. Sometimes it means a specific cryptocurrency (“the Bitcoin blockchain”). Enterprises have spent a decade trying to use “blockchain” to mean “a database that several companies can write to”, which is a real thing but has very little to do with what people mean when they say “blockchain” in the context of Bitcoin.
Why the Chain Part Matters
The important property is that a blockchain is append-only and globally agreed. Every participant in the network holds the same history, and new blocks are added to the end by some consensus mechanism — proof-of-work in Bitcoin, proof-of-stake in Ethereum, a variant of either in most other chains. Nothing can be removed or edited, only added. If the consensus rules are good and the network is large, this gives you something that no previous kind of database ever gave you: a record that is authoritative without being controlled by anyone.
That property is the entire point. The cost is enormous — a distributed ledger with global consensus is several orders of magnitude slower and more expensive than a centralised database, and the trade-off only makes sense if you actually need the trust-minimisation property. For a bank or a logistics company running an internal system, you do not need it. For an international currency that nobody owns, you do.
Not All Blockchains Look the Same
The data structure varies more than you might think. Bitcoin uses a pure chain of blocks. Ethereum uses a chain of blocks whose contents are a state tree (the balances and contract storage at a point in time). Solana batches transactions in a way that overlaps blocks partially. Some projects have experimented with DAGs (directed acyclic graphs) that technically are not chains at all but tend to get bucketed in anyway. What they all share is the property of being a sequential, append-only, cryptographically linked history agreed on by a decentralised set of participants.
Everything that makes crypto interesting — the transparency, the immutability, the auditability, the ability to have programs that run forever with nobody in charge — flows from that.