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What is a fork?
A fork is a change to a blockchain's rules, or a split where two versions of the chain continue. A soft fork stays compatible with older software; a hard fork does not, and if some users reject it, two separate chains result, as with Ethereum and Ethereum Classic.
What it means for you
After a split, people who held coins at that moment usually hold them on both chains, but each exchange and wallet decides separately whether to support the new coin. A site asking for your seed phrase to 'claim' fork coins can take everything on both chains.
How it works
The word covers two things. A temporary fork happens when two miners find blocks at the same height at nearly the same time; nodes follow the chain that took the most work and the other block goes stale. A protocol fork changes the rules. A soft fork only makes some previously valid blocks or transactions invalid, so old nodes still accept new blocks; a hard fork changes the rules so non-upgraded nodes cannot validate upgraded blocks, and if both sides keep running, the chain splits permanently. Ethereum's July 2016 fork at block 1,920,000 moved funds lost to a faulty contract; miners who refused kept the old chain going.
Sources: Bitcoin developer glossary, ethereum.org: The history of Ethereum, Bitcoin developer guide: Block chain · checked 4 October 2026
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