What is a blockchain? (My 1st steemit post ever)

It seems like many people have a problem concisely explaining exactly what a blockchain is. Here are my 3 go-to phrases when I am trying to sum it up as quickly as possible.

What is a blockchain?
A blockchain is a decentralized database  (aka “a ledger”).

What is a decentralized database (aka “a ledger")?
A decentralized database is a world-wide computer that is formed by many individual computers communicating with each other. As a result the record cannot be altered retroactively without the alteration of all subsequent blocks and the collusion of the network.

Why do we want to use blockchain/decentralized database technology?
The key feature is the ability to eliminate having to put our trust in the hands of manipulative centralized institutions such as Facebook, YouTube, Google, banks, media conglomerates, etc., and instead put our trust in decentralized solutions that rely on mathematics and computing  (which are much less likely prone to errors).

How would you describe what a blockchain is, and what the technology can be used for?
Can you explain it better or more concisely?
Let me know in the comments!