Is Steem a Blockchain?

Could you please clarify for me whether Steem is indeed a blockchain? I've been hearing a lot about it lately, and I'm curious to know if it's part of the decentralized, distributed ledger technology that's becoming increasingly popular in the cryptocurrency and finance world. I understand that blockchain technology is known for its transparency, security, and immutability, so I'm wondering if Steem shares these same characteristics. Your insights would be greatly appreciated.