"blockchain" is a term only used in the context of cryptocurrencies where a chain of transaction blocks is agreed upon in a distributed system. In any other case it's called a merkle tree. The core tech which distinguishes blockchain is not the series of signatures which provide proof of integrity (and perhaps more important in cryptocurrency: that they were derived from the previous part of the chain. This doesn't matter for data integrity per se, but it matters for consensus rules), but the consensus rules which say 'this chain is the one chain which counts, not the others'.

Hence it is and should be named DLT. The "Distributed" separates it from merkle tree systems.

Except that there are also "permissioned blockchains" which are non-distributed systems. Of course those are more an abuse of terminology to hype various services, but the term has appeared in the research literature. The consensus algorithm in such a blockchain is just "whatever blocks the controlling entity accepts into the canonical branch".

Permissioned in that case simply means private (you need a permission to participate). It does not mean non-distributed. It also doesn't mean any entity with permission can add anything and its accepted. That would just be a shared/synced database. A DTL still has rules and all participant collectively enforce them. Double spend is still prevented by the same technology. There must be some form of central authority to make it private but DLT limits the power of that authority, it can maybe add or remove participants and such stuff but it can not change the rules. Every participants would have to agree to run the new rules

This is certainly useful to connect entries with shared balanced like banks who cooperate. Every bank still wants to validate for themself and not blindly trust whatever another bank writes on the ledger. Moving balanced form someone else or double spend is not possible in such a system, even trough all have the same permissions.

A permissioned/private "blockchain" is still a DTL as long as the consensus enforcement is still distributed. If all participants have to trust all others to simply follow the rules then its not a DTL anymore.

This is where the conversation gets tricky, though. If, as the bit rot example would imply, blockchain is just a fancy word for hash chain, then that statement is uncontroversial. Hash chains their cousins have been around for something like half a century and are well established.

That's really hard to reconcile with the "blockchain is revolutionary" meme, though. I think you've got to include distributed consensus and proof of work in the definition in order to get one that characterizes blockchain as a new invention and not just marketerspeak. Unfortunately, that definition doesn't jibe very well with the usual lists of successful uses of blockchain outside of cryptocurrency, such as corporate supply chain management, because they tend to be using blockchain where a hash chain would work just as well. Perhaps they're actually using hash chains, and just using the word "blockchain" as marketerspeak. I don't have inside knowledge and couldn't evaluate such things, but it wouldn't surprise me at all if the more commonly touted examples were doing so because distributed consensus is not a desirable thing for the problem they're trying to solve.

In the abstract, the problem is that it gets really hard to understand what everyone is actually talking about when the conversation centers on a popular buzzword.

I suspect what you're calling a blockchain is actually a merkle tree. The blockchain is a merkle tree structure shared via p2p communication over a protocol designed to prevent double spending.

Merkle trees were invented in 1979. If companies looking to prevent data tampering didn't make use of them until they got rebranded as "blockchain" then those companies are just bad at what they do.

> The blockchain is a merkle tree structure shared via p2p communication over a protocol designed to prevent double spending.

I'd say "The blockchain is a Merkle tree structure with a consensus algorithm which selects a canonical path to a leaf node at any given time."

I don't think preventing double spending is a meaningful concept for non-cryptocurrency blockchains, since those have nothing to spend. Whether we accept that such things are blockchains determines whether this distinction makes any sense. Though, if all blockchains are cryptocurrencies then there's not really any need to separate the two terms.

Yes you're right, that's a better generalised description of them. Of course that's what I meant by "double spending" (essentially, the list becoming a graph, ie forking), but precision in communication is important.