What property describes a blockchain's unchangeable record?

The property that describes a blockchain’s unchangeable record is immutability.

In blockchain systems, transactions are grouped into blocks, and each block contains a cryptographic hash linked to the previous block. Once later blocks are added, changing an earlier transaction would require altering the chain that follows it and persuading the network to accept the replacement.

Immutability does not mean that blockchain data is magically impossible to change under every circumstance. It means validated records are designed to resist retroactive alteration. The practical strength of that resistance depends on the network’s consensus rules, the number and independence of its participants, and the cost of controlling enough computing power or voting power.

Transparency is a different idea: it concerns who can inspect the ledger. Decentralization describes how control is distributed, while consensus is the process nodes use to agree on valid updates. Those features help support immutability, but they are not synonyms for it.