A blockchain is a form of distributed ledger that organizes data into blocks and features an append-only structure, i.e. nothing can be edited or deleted after the fact.
The Bitcoin system was the first instance of a cryptocurrency blockchain, i.e. its design prevents Alice using the same single digital token to pay both Bob and Charlie (double-spending).
Ethereum is the first blockchain to feature a cryptocurrency and smart contract functionality.
There is a wide variety of blockchain designs.
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