Solidity is a smart contract language primarily used for Ethereum. While Cairo is designed for verifiable computation and general-purpose computation on StarkNet, a layer 2 scaling solution for Ethereum, both languages serve the purpose of writing smart contract logic. Thus, they can be considered competitors within the smart contract development space, especially for developers choosing a language to build on Ethereum-related ecosystems. However, Cairo focuses more on proving computation, making it different from Solidity in some respects.