All terms
An independent chain connected to another by a bridge, with its own security.
A sidechain runs its own consensus and validator set. It connects to a main chain through a bridge but does not inherit its security.
This is the crucial difference from a rollup. If a sidechain's validators collude, the main chain cannot help you.
Polygon PoS began as a sidechain. That is why L2BEAT categorizes it differently from true rollups.
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