Blockchain Fundamentals

51% Attack

Also: Majority Attack

When one party controls more than half of a network's mining power (or stake) and uses it to rewrite recent history, typically to double-spend. The bigger and more distributed the network, the more expensive this is. Small chains have been hit by it; Bitcoin never has.

General education, not investment, legal or tax advice. Markets and regulation change quickly: where a definition depends on current law or on a recent event, check the latest coverage before relying on it.