The term used to describe a blockchain network or protocol that is fully operational.
Mainnet is a crypto-related term for blockchain networks, projects, platforms and protocols that have already started operations. This is not the kind of network that is still being developed or still sending out invitations for investments. Rather, mainnets are defined by the fact that the platform is fully operational, traders and miners are active on the network, and transactions are ongoing, all of which are being recorded on the blockchain.
In a way, mainnets are the desired destinations of testnets. Testnets are simply working samples still subject to rigorous testing by the developers and community alike until a fully working model is released. Some blockchain platforms (e.g. Solana) are already mostly functional at testnet while others (e.g. Radix) only add critical functionalities such as smart contracts at the mainnet stage.