Sidechain
An independent blockchain that runs alongside a main chain and connects to it through a two-way bridge.
A sidechain is its own blockchain with its own consensus and validators. It connects to a main chain like Ethereum through a bridge, but it does not inherit that chain's security. Polygon PoS is the best-known example.
That is the key difference from a rollup. A rollup posts data to Ethereum and borrows its security; a sidechain stands alone. If its validators are compromised, the main chain cannot save you.
The appeal is speed and low fees. The risk is a smaller, separate security model, and a bridge that hackers love to target.
Related terms
A protocol that moves tokens or data between two separate blockchains that cannot otherwise talk to each other.
Layer 2A separate network built on top of a base chain to process transactions faster and far more cheaply.
RollupA scaling method that batches many transactions off-chain and posts a compressed proof or record to the base layer.
Layer 1The base blockchain itself, where transactions settle and security lives, before any add-on networks built on top.