Definition
The longest chain rule (more precisely, the heaviest chain rule) is how Bitcoin nodes determine which chain of blocks is the valid one. Nodes always follow the chain that has the most accumulated proof of work, which in practice means the chain with the most blocks. This rule resolves temporary forks when multiple valid blocks are found near-simultaneously.
This mechanism is fundamental to Bitcoin’s security: an attacker would need to outpace the rest of the network’s hashrate to create a longer competing chain. Combined with the difficulty adjustment, this makes Bitcoin’s transaction history increasingly immutable over time.
In Simple Terms
Nodes accept the chain with the most proof of work as the valid blockchain. Resolves competing chains.
The consensus rule where Bitcoin nodes accept the chain with the most accumulated proof of work (most computational effort) as the valid blockchain.