Sidechain: a separate chain bridged to a main one

Connected But Independent

A sidechain is an independent blockchain that runs in parallel to another (often Ethereum) with its own consensus, validators, and block production. Tokens move between the two through a bridge, but the sidechain does not inherit the security of the main chain the way a rollup does. If the sidechain's validators collude or fail, the chain can be compromised regardless of what happens on the main chain.

Polygon PoS is the most prominent example next to Ethereum—a fully separate proof-of-stake chain with its own validator set and faster, cheaper transactions. Liquid and Rootstock connect to Bitcoin in similar ways. Sidechains typically offer lower fees and higher throughput than the chain they sit next to, at the cost of trusting their own consensus layer rather than the parent's.

The line between sidechain and Layer 2 has blurred over time, but the distinction is real. A rollup posts data and validity guarantees back to L1; a sidechain just runs its own consensus and trusts a bridge contract to mirror assets. For users, this means thinking about who validates the chain you are on and what assumptions you are making about that validator set's honesty—not just looking at the marketing label.