A testnet is a test network used in blockchain development. A testnet is a "sandbox" environment used to run and test blockchains and blockchain projects before they are released. Some testnets are used before the release of the corresponding mainnet, or main network, and some are used after a mainnet is in production. Testnets used prior to the release of the mainnet are environments that are being used to create and test a mainnet before it goes live, whereas testnets used after a mainnet is live are used to improve a mainnet and test decentralized applications.
A testnet is a live environment in which application developers can test their decentralized applications and blockchain developers can test upgrades to the underlying platform. The testnet is a similar environment to the mainnet and allows developers to test code with no risk of disrupting the mainnet production network. Applications and code on testnets are expected to break as part of the testing process, and the tokens on testnets are considered worthless. Developers can get test tokens via faucets, which are web-based services that provide free tokens to users of the testnet.
Intro to Testnets by James Bachini
Blockchain networks can maintain multiple testnets for various purposes. For example, Ethereum has five testnets: Ropsten (a deprecated testnet, was a proof-of-work testnet that went through The Merge to proof-of-stake), Sepolia (proof-of-work testnet), Goerli (proof-of-authority testnet), Rinkeby (deprecated), and Kovan (deprecated), and Bitcoin has had three generations of testnets—testnet, testnet2, and testnet3.
