# Blockchain

By [Amir](https://paragraph.com/@amirhr3) · 2022-08-08

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A **blockchain** is a type of [Digital Ledger Technology (DLT)](https://en.wikipedia.org/wiki/Distributed_ledger) that consists of growing list of [records](https://en.wikipedia.org/wiki/Record_\(computer_science\)), called _blocks_, that are securely linked together using [cryptography](https://en.wikipedia.org/wiki/Cryptography).[\[1\]](https://en.wikipedia.org/wiki/Blockchain#cite_note-fortune20160515-1)[\[2\]](https://en.wikipedia.org/wiki/Blockchain#cite_note-nyt20160521-2)[\[3\]](https://en.wikipedia.org/wiki/Blockchain#cite_note-te20151031-3)[\[4\]](https://en.wikipedia.org/wiki/Blockchain#cite_note-cryptocurrencytech-4) Each block contains a [cryptographic hash](https://en.wikipedia.org/wiki/Cryptographic_hash_function) of the previous block, a [timestamp](https://en.wikipedia.org/wiki/Trusted_timestamping), and transaction data (generally represented as a [Merkle tree](https://en.wikipedia.org/wiki/Merkle_tree), where [data nodes](https://en.wikipedia.org/wiki/Node_\(computer_science\)) are represented by leafs). The timestamp proves that the transaction data existed when the block was created. Since each block contains information about the block previous to it, they effectively form a _chain_ (compare [linked list](https://en.wikipedia.org/wiki/Linked_list) data structure), with each additional block linking to the ones before it. Consequently, blockchain transactions are irreversible in that, once they are recorded, the data in any given block cannot be altered retroactively without altering all subsequent blocks…

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*Originally published on [Amir](https://paragraph.com/@amirhr3/blockchain)*