Glossary
Mainnet
The production, real-money instance of a blockchain — as opposed to test networks whose native asset has no value.
Mainnet is where actual value moves. Token balances are real, validators are paid in valuable native assets, and fees come out of real wallets.
Most chains also operate one or more testnets — same client software, no economic stakes — for developers to deploy and test before going live. A "mainnet launch" is when a project graduates from testnet and starts behaving as a production system. Mainnet upgrades are correspondingly cautious affairs.