A change to a blockchain's rules. Soft forks are backward-compatible; hard forks create a separate chain.
A fork occurs when a blockchain's protocol rules are changed. There are two types: a soft fork is a backward-compatible upgrade where nodes running old software can still participate (like tightening rules), while a hard fork is a non-backward-compatible change that creates a permanent split into two separate chains if not all participants upgrade. Contentious hard forks happen when the community disagrees on direction — Bitcoin Cash forked from Bitcoin in 2017 over the block size debate, and Ethereum Classic continued the original Ethereum chain after the DAO hack was reversed on Ethereum. Planned hard forks (like Ethereum's Shapella upgrade enabling staking withdrawals) are routine upgrades where everyone agrees to move forward. The term also applies to software forks — SushiSwap famously forked Uniswap's code to create a competing DEX.
A fork occurs when a blockchain's code is modified, creating a divergence from the original chain. Soft forks are backward-compatible upgrades that tighten rules (nodes running old software still recognize new blocks), while hard forks change rules so fundamentally that old and new versions become incompatible — creating two separate blockchains. The most notable hard fork created Bitcoin Cash (BCH) from Bitcoin in 2017 over a disagreement about block size. Ethereum's hard fork after the 2016 DAO hack created Ethereum Classic (ETC). Not all forks are contentious — routine protocol upgrades like Ethereum's Shapella upgrade are planned hard forks where the entire community moves to the new version. The term 'fork' also applies to code: anyone can fork (copy) a blockchain's open-source code and launch a modified version, which is how many 'Ethereum killers' and DeFi protocol clones originated.
Bitcoin Cash forked from Bitcoin on August 1, 2017, creating a separate blockchain with 8MB blocks (vs Bitcoin's 1MB) — holders of Bitcoin at the time received equal amounts of both BTC and BCH.
In contentious hard forks where both chains survive (like Bitcoin/Bitcoin Cash), holders of the original token typically receive equivalent amounts on both chains. However, accessing forked coins often requires technical steps, and many forks produce tokens with little lasting value. Don't buy a cryptocurrency solely for a potential fork airdrop.