I should use that in my Bitcoin lectures, as it's the most complete definition of consensus ever.
"Consensus" in Bitcoin is one word doing seven jobs. Almost every fight you've watched is two people using different definitions, each assuming the other is arguing in bad faith.
Consensus is not agreement. It's a wide range of self-interested actors, node runners, miners, exchanges, and developers, each doing what they believe is in their own economic interest and happening to converge on the same chain. It's emergent, not decided. It can't be forced.
Here are the seven meanings.
1️⃣ Consensus (Dictionary). General agreement. This is the definition Bitcoin has never satisfied and doesn't need to. The entire invention is producing convergence without agreement: a room that never took a vote, somehow still ending up in the same building. If your argument requires everyone to agree, you've misunderstood the machine.
2️⃣ Consensus (Rules). The validity conditions every node independently enforces. 21 million cap. No double spends. Valid proof of work. Script rules. These are inviolable at runtime: break one and you've forked yourself off, alone, holding 100% of nothing. They are not immutable. P2SH, CLTV, CSV, SegWit, and Taproot all changed the set. Binding, not permanent.
3️⃣ Consensus (Nakamoto). The valid chain with the most accumulated work. Notice the division of labor: the rules decide which chains are valid; Nakamoto consensus decides which valid chain is the chain. A filter, then a tiebreaker. It's probabilistic. Never final, only expensive to reverse.
4️⃣ Consensus (Rough). Borrowed from the IETF. RFC 7282 defines it precisely: consensus is reached when all issues have been addressed, not necessarily accommodated. You can hate a change, say so loudly, get a real technical answer, and rough consensus still exists. It is not unanimity and it is not a vote. The catch is that the IETF has chairs with the standing to declare it. Bitcoin has nobody. We imported the process and left the referee behind. That gap is where most of the shouting lives.
5️⃣ Consensus (Activation). The mechanical thresholds. Soft forks have typically required 95% of blocks in a 2,016-block period to signal, 1,916 out of 2,016. This looks like voting. It isn't. Those signaling bits were never able to reject a block as invalid. Miners signal readiness to enforce; nodes do the enforcing. Mistaking signaling for suffrage was the original sin of 2017.
6️⃣ Consensus (Economic). The actors who actually price the chain: holders, exchanges, custodians, and merchants. 2017 settled this one empirically. A hashrate majority signed a document committing to 2x. It never shipped. The market simply declined to call it Bitcoin. Hashrate orders blocks. It does not define money.
7️⃣ Consensus (Implementation). The ugliest one. There is no formal specification. The running code is the spec, bugs included. In March 2013 the network split because a block that new nodes accepted, older nodes rejected. The new chain had most of the hashpower, and pools deliberately downgraded to abandon it and stay bug-for-bug compatible with the old software. Consensus that day meant "whatever the deployed code actually does."
One bonus distinction that would end half the current arguments: consensus is not policy. Different rules about what you accept in a block will fork you off. Different rules about what your node relays will not. Most of today's drama is policy wearing a consensus costume.
So when someone tells you "there's no consensus for this," ask which one.
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