A situation where a blockchain splits into two separate chains. Forks generally happen in the crypto-world when new ‘governance rules’ are built into the blockchain’s code, therefore giving birth to a new type of coin. Each fork contains all the old blocks in the chain, but after the split they each act as their own blockchain.
Examples: BitcoinCash and BitcoinGold forked from the original Bitcoin blockchain.
This has caused controversy among crypto enthusiasts because not only does it call into question which is the “real” original chain, but it also raises a question of whether or not the split effectively doubles the amount of coins (this was very much the case when Bitcoin forked to create Bitcoin Cash).