A blockchain is a distributed ledger that a network of nodes maintains together. Each block stores transactions and a cryptographic hash of the previous block. That chain of hashes is what makes history hard to rewrite. Change one block and its hash changes. The next block still points at the old hash, so the link breaks. Repair that next block and you break the block after it.
To rewrite history you would need to recompute every later block, and you would need to do it faster than the live network adds new ones. On established chains that work is computationally infeasible.
Bitcoin and Ethereum are blockchains. They record who sent what to whom on this shared, append-only ledger. Consensus is the protocol that lets untrusted nodes agree on what the ledger contains without a central operator. Anyone can download the chain and verify that a given transaction exists. No single entity controls it. Past transactions cannot be changed once enough later blocks sit on top.
Those properties matter when parties need a shared source of truth and do not trust each other. Uses beyond currency include smart contracts, identity checks, supply chain records, property rights, and voting. This chain of hashes creates immutability. The blockchain is transparent.
Satoshi Nakamoto's 2008 Bitcoin paper described a timestamped chain of hashed blocks so money could move without a bank.
Blockchain Immutability
Edit a block to see how changing data breaks the chain's integrity
Try editing any block's data to see how it breaks the cryptographic chain. This demonstrates why blockchain is immutable - changing history requires recalculating all subsequent blocks.