Blockchain is just a data structure.

Strictly speaking a blockchain is just a data structure. However, many people (including me on rare occasions) use the word blockchain to describe a peer-to-peer network with a consensus algorithm.

A blockchain is a chain of blocks. Each block contains at least a timestamp, the actual data, and a cryptographic hash of the previous block. Since each block contains a reference to the previous block, the blocks are “chained.” Each block hash is calculated based on the timestamp, the data, and the hash of the previous block.

A blockchain is a great data structure for distributed systems where data integrity is important because if there is a change in one block then all subsequent blocks need to change, otherwise the chain “breaks¹.”

Summary

A blockchain is just a data structure. A blockchain can exist without a peer-to-peer network and without any consensus algorithm.

Footnotes

1. Unless there is a cryptographic hash collision. Then we have a problem! ↩