A blockchain is a database of blocks which are cryptographically hashed such that manipulating any previous entries would invalidate all future ones referencing

A blockchain is a database of blocks which are cryptographically hashed such that manipulating any previous entries would invalidate all future ones referencing it. That is pretty much it. This can be constructed on a single machine in someone's basement if one so desires. In fact, the entire point of Bitcoin is that the entire blockchain can be validated by any node which anyone can start with something as trivial as a raspberry pi. Now Bitcoin adds a decentralized mechanism on top of this using Proof of Work. But that is not inherent to blockchains.