A Layer 2 (L2) network is a system built in relation to a Layer 1 blockchain. In Ethereum’s documentation, the term covers particular scaling solutions that handle transactions separately from Ethereum Mainnet while deriving security directly from Ethereum’s Layer 1 consensus.
Rollups are one type of Layer 2
Ethereum documentation describes rollups as an L2 approach. Rollups execute transactions outside Ethereum’s L1 and submit data or results to Ethereum. Optimistic and zero-knowledge rollups use different validation approaches, so “rollup” is not a synonym for every L2 system.
Layer 2 is not the same as a sidechain
Ethereum’s documentation distinguishes L2 systems from sidechains. A sidechain is a separate blockchain with its own consensus and security arrangements, while an L2 is defined by its relationship with the underlying L1.
In short
A Layer 2 network is built in relation to a Layer 1. Ethereum rollups are examples, but not every system described as a separate chain should be called an L2.