Testnet
A copy of a network used for testing, where the coins have no market value and mistakes cost nothing.
Developers deploy contracts, exercise upgrades, and run load tests on a testnet before touching the live network, and testnet coins are handed out free by faucets so anyone can experiment. Testnets often use a cheaper consensus arrangement, such as a permissioned set of block producers, and they are periodically retired and replaced as they accumulate junk state. Ethereum has run a succession of public testnets, including Sepolia and Holesky, alongside short-lived devnets used to rehearse specific upgrades. Readers meet testnets in developer documentation, in bug bounty scopes, and in announcements that an upgrade has been tested before mainnet activation.
In practice
Before a network upgrade, client teams activate it on public testnets first so that bugs surface where no real assets are at risk.
The common misunderstanding
Testnet tokens are not a free version of the real asset; they are issued on request by faucets, hold no value, and cannot be moved to mainnet.