Blockchain is a record of events that are shared between many parties, and once information is entered, it cannot be altered as the downstream chain reinforces upstream transactions.
Any transaction or data exchange is recorded permanently. It does not reside on a single centralised server, in contrast, it is decentralised, managed by distributed nodes. All nodes have a copy of the entire blockchain, and by distributing copies and access, the chain cannot simply “go down,” or disappear making it secure. It is used in not only cryptocurrency applications but more recently as a data storage and record keeping.