A block chain is a transaction database shared by all nodes participating in a system based on the BitCoin protocol. A full copy of a currency's block chain contains every transaction ever executed in the currency. With this information, one can find out how much value belonged to each address at any point in history. https://en.bitcoin.it/wiki/Block_chain
May'2014: Minimum Viable Block Chain, by Ilya Grigorik. The block chain is agnostic to any "currency". In fact, it can (and will) be adapted to power many other use cases. As a result, it pays to understand the how and the why behind the "minimum viable block chain".
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