“Blockchain” nowadays is referring to “Blockchain technology”, I put statements as below for its concise definition:
From technical perspective, “Blockchain” is linked-list data structure. One of its applications is to store data history. (The linking can be for many other purposes, not only for data history. But here we concentrate on talking about data history storage for ledger purpose.)
In a linked-list, the latest data block links to its previous block by including hash value of previous block in its own data section. Then the system generates hash value of that latest data block using cryptographic algorithm e.g. SHA-256, for tamper-evident purpose.
The linked-list can be “practically” immutable when it runs on top of a decentralized network (which has no centralized decision maker) with consensus mechanism, e.g. proof-of-work, which is to replace the trust among the network nodes.
We say “practically” because the consensus mechanism can be broken under certain circumstance, e.g. for Bitcoin, the breaking scenario is 51% attack.
For “distributed ledger” implementation, a linked-list is copied across network to network nodes serving as a distributed database. Thus, everyone can own a copy of the ledger linked-list for audit trail purpose.