Sidechain
An independent blockchain connected to a main chain by a bridge, with its own security — related to but distinct from a rollup.
A sidechain is a separate blockchain that runs in parallel to a main chain and connects via a bridge, but crucially provides its own security (its own validators or consensus) rather than inheriting the main chain’s. Polygon’s original PoS chain is a well-known example.
This is the key distinction from a rollup: a rollup posts data and proofs to inherit Layer 1 security, while a sidechain is trust-independent — if its own validator set is compromised, the main chain can’t protect it. Sidechains offer speed and low fees but require trusting their separate security model and the bridge connecting them. The difference matters: “secured by Ethereum” (rollup) and “connected to Ethereum” (sidechain) are very different guarantees.