A Sidechain is a secondary Blockchain connected to the main blockchain with a two-way peg. Sidechains may have their own consensus protocols, which could be completely different from the mainchain's protocol. Theoretically, a sidechain can add new functionalities, improve privacy, and security of traditionally vanilla blockchains.
Source: https://www.sciencedirect.com/science/article/abs/pii/S1084804519303315