What is a blockchain?

A blockchain is a decentralized database and infrastructure, run by the users, for the users, that requires a cryptocurrency to truly operate without a central party and/or intermediaries. For example, the $FEVR token is deployed on the BNB Smart Chain, with bridges to other blockchains: Ethereum, Polygon, Avalanche, etc...
Now let's pick this definition apart and put it in simpler terms!
It's a network system where transactions are individually stored in the form of blocks linked to each other, and can never be modified. This is how the term "blockchain" came to be, since it is essentially a chain of linked blocks.