Glossary
Sidechain
An independent blockchain that runs in parallel to a main chain and connects via a bridge, with its own validators and security assumptions.
A sidechain is its own blockchain — its own consensus, its own validators, its own block times. It connects to a main chain via a bridge so assets can move between them, but it does not inherit the main chain's security the way a rollup does.
In practice that means a sidechain is only as safe as its own validator set. Polygon PoS is the most-used example: cheaper and faster than Ethereum, with a separate security model that users have to accept.