What is Optimistic Rollup (OR)?

An Optimistic Rollup (OR) is a type of Layer-2 scaling solution that runs on top of the Ethereum network. Optimistic Rollups are similar to the Plasma scaling implementations used for Ethereum, but in contrast to Plasma, Optimistic Rollups allow for the operation of an Ethereum Virtual Machine (EVM) compatible Optimistic Virtual Machine (OVM), which enables increased functionalities for Optimistic Rollup technology. The name 'Optimistic Rollup' refers to how 'optimistic' aggregators tend to run without committing fraud and instead provide proofs in case of fraud, while 'rollup' is from the process of implementing transaction bundles that are 'rolled up' and that thus allow network transaction speeds to increase exponentially.