It has occurred to me that when people are talking about Bitcoin, they are often talking about the current technical implementation. I don't think this is the right way to define it, as it stops being a protocol and starts being 'bitcoind' almost by accident. It also in many ways hinders Bitcoin from evolving should that be necessary. I'm going to propose a complete definition of Bitcoin here that does not rely on any technical implementation specs, but which is both currently accurate to the existing protocol, and flexible enough to allow for significant changes over time. Would love feedback.

Bitcoin is:

• A proof of work blockchain
• with a hard-capped supply limit of 21 million units (further subdivision is allowed but the decimal place must remain fixed)
• the ledger of which includes in the record the 2009 genesis block
• the verification nodes of which have a performance cost low enough that the hardware requirements can be met by anyone with a desire to run one
• is decentralized enough that node operators, not miners, software developers, governments etc., are the final arbiters of valid blocks
• and finally has the widest consensus level

An important caveat here is that a blockchain that fully complies with the first 5 rules is a contender for being Bitcoin, but the final rule of majority consensus is what determines the title winner.

The blockchain that everyone has thus far called Bitcoin fits this definition perfectly and is the only chain to do so. Prior hard-forks were, at least temporarily, bitcoin contenders, but as they never achieved majority consensus and thus were never Bitcoin. A future hard-fork may well gain such majority consensus: that would make the hard-fork Bitcoin, and the original protocol the failed fork. It should be rather easy to determine in a hard-fork event which protocol is Bitcoin: the failed fork will be rapidly abandoned and/or sold off. The primary 5 rules also allow node operators to determine whether a proposed change to the protocol can even qualify as Bitcoin, and dismiss without further consideration any proposal that doesn't qualify as a contender.

The primary 5 rules here strike me as essential components of Bitcoin as a viable monetary technology. There are perhaps elements that are similarly essential that I am not considering, though I am deliberately keeping the essential list to what are critical essentials. Many, many potential contenders could exist within this paradigm, but without consensus they are just failed forks; as such there is no need to cover multiple edge cases as the free market will deal with them.