Blockchain
Also known as: distributed ledger, chain
A shared, append-only ledger of transactions maintained across many computers, providing the immutable record behind most crypto assets.
Definition
A blockchain is a distributed ledger where transactions are grouped into blocks and chained together cryptographically, replicated across many nodes so no single party controls it. Its immutability and public visibility (on most chains) mean transaction histories are permanent and traceable. For tax, this permanence is double-edged: it gives you a complete record to reconstruct base cost, and it gives authorities a durable audit trail.
Example
Every BTC transaction sits on the Bitcoin blockchain, so years later you can trace the exact transfers needed to rebuild your base cost.