The blockchain is a technology of storage and transmission of information, transparent, secure, and functioning without a central autority.
By extension, a blockchain is a database that contains the history of all the exchanges made between its users since its creation. This database is secure and distributed: it is shared by its different users, without intermediaries, which allows everyone to check the validity of the blockchain.
There are public blockchains, open to all, and private blockchains, whose access and use are limited to a certain number of actors. A public blockchain can therefore be likened to a large public accounting book, anonymous and unfalsifiable.