A sidechain is an independent blockchain network that runs in parallel with a main network (such as Ethereum or Bitcoin) and is connected to it through bidirectional bridges. The main goal of sidechains is to offload operations from the main network, increasing scalability and reducing transaction fees by performing some operations outside the primary blockchain.

Key features of sidechains:

Connection to the main blockchain: Assets can move between the main network and the sidechain via bridges.
Autonomy: Each sidechain has its own consensus rules, separate from the main blockchain.
Scalability: Sidechains help increase the throughput of the network, improving scalability.

Example: Polygon is one of the most well-known sidechains of Ethereum.