Mainnet
The live network where transactions carry real value and are permanent, as opposed to a practice network.
A mainnet launch is the point at which the genesis block is produced and real assets begin to move; before that, the software exists only on test networks. Because mistakes on mainnet cost real funds and cannot be rolled back without coordinated intervention, upgrades are rehearsed on testnets and then activated at an agreed block height or epoch. Contract addresses, token contracts, and bridge deployments are all specific to a given mainnet, which is why sending an asset to the right address on the wrong network loses it. Marketing phrases such as mainnet beta have no technical definition.
In practice
Ethereum's Merge in September 2022 moved mainnet from proof of work to proof of stake after the same transition had been rehearsed on several public testnets and shadow forks.
The common misunderstanding
A mainnet launch is a technical milestone, not a verdict on quality; a live network can still be incomplete, centrally controlled, or barely used.