Glossary
The DAO Hack (2016)
The June 2016 reentrancy exploit that drained ~3.6M ETH from "The DAO" — led to the Ethereum / Ethereum Classic chain split via a contentious hard fork.
"The DAO" was a 2016 Ethereum-based investment DAO that raised ~12.7M ETH from over 11,000 contributors. On June 17, 2016, an attacker exploited a reentrancy bug in The DAO's withdrawal function to recursively drain ~3.6M ETH (worth ~$50M at the time, ~$5–10B at later peaks).
Ethereum's developers and a majority of the community elected to hard-fork the chain to roll back the theft. A minority refused — that chain continued as Ethereum Classic (ETC), with The DAO's balance intact for the attacker. The split is the most important event in Ethereum's governance history and a permanent reference in any conversation about chain immutability.