Fusion Rollups

Fusion rollups is a loosely used term for rollup designs that blend more than one scaling technique, most often the dispute-based security of an optimistic rollup with the cryptographic proofs of ZK-rollups. The general idea, also called a hybrid rollup, is to run cheaply under optimistic assumptions most of the time while using zero-knowledge proofs to shorten withdrawal delays or settle disputes faster. The phrase is sometimes also applied to schemes that link several rollups so they can share liquidity and messages. It is not an established technical standard, and different projects use it for different designs, so readers should check what a particular project means by it. The broader category is described under layer 2.