A mainnet is the functional blockchain network where actual transactions occur, distinguishing it from test versions or experimental networks.
A mainnet, short for “main network,” is the primary blockchain network where actual transactions occur, and the native cryptocurrency is used.
It is not the same as testnets or experimental environments; instead, it is the product of development activities. The mainnet is distinguished by the use of its own cryptocurrency, a digital asset essential to the network’s operation. This native currency is vital to the ecosystem’s governance procedures, staking schemes and transaction fees.
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Source: https://cointelegraph.com/explained/what-is-a-mainnet-in-cryptocurrency-explained