What Is a Sidechain and How Does Its Security Model Differ from a Rollup?

A sidechain is a separate, independent blockchain that runs parallel to the main chain (L1) and is connected by a two-way bridge. Its security model is independent of the L1; it relies on its own set of validators and consensus mechanism.

A rollup, conversely, is a Layer-2 solution that derives its security directly from the L1. If a rollup's sequencer is malicious, the L1 can enforce the correct state via fraud or validity proofs, which is not the case for a sidechain.