There is no consensus definition of what a blockchain is. In fact the Satoshi whitepaper that introduced Bitcoin does not even use the word blockchain it talks about a hash chain of blocks a concept which predates Bitcoin by ~30 years. The real innovation of Bitcoin was to use PoW to both limit Sybil attacks and to make additions to the hash chain computationally expensive to revert. Often however when people talk about blockchains they are talking about systems which don't use PoW at all. Like end-to-end encryption or peer-to-peer networks blockchain describes a cloud of concepts and technologies. A better terminology would be DLT (Distributed Ledger Technology).