Mainnet

Mainnet is the main public network of a blockchain, where tokens and transactions carry real value, as opposed to a testnet used for trials. ethereum.org defines Ethereum Mainnet as the primary public production blockchain, where actual-value transactions occur, and notes that when people and exchanges discuss ETH prices they mean Mainnet ETH. There is one Mainnet, while any number of testnets can copy its rules for testing.

Layer 2 networks have mainnets of their own, and launch dates can matter to airdrops. Arbitrum's airdrop rules counted points earned before the Nitro upgrade went live on Arbitrum One mainnet, at block 22207817 on 31 August 2022, at double value, and stated that ARB could be claimed only on Arbitrum One, not on Ethereum Mainnet. A claim page that asks you to sign on a different network from the one the project names is a reason to stop and check.