A mainnet (main network) is the fully operational, live version of a blockchain where actual cryptocurrency transactions occur and have real economic value. It's the production environment as opposed to test networks.
Mainnet vs Testnet
Mainnet: Real transactions, real value, real consequences. This is where your actual cryptocurrency lives.
Testnet: A sandbox environment with no real value, used by developers to test applications and upgrades before deploying to mainnet.
Mainnet Launch
A "mainnet launch" is a significant milestone for any blockchain project. It marks the transition from a concept or testnet to a fully functional, live network. Many projects operate on testnets or as tokens on other blockchains before launching their own mainnet.
Examples
Bitcoin's mainnet launched on January 3, 2009. Ethereum's on July 30, 2015. Solana's on March 16, 2020. Each of these dates marks the moment the network went live and began processing real transactions.