Cosmos ATOM: Stargate Testnet
"Block explorers, wallets, exchanges, we are asking you to schedule time in August for integration testing against the Stargate network.
Validators, we are hoping that nearly all validators in the Cosmos Hub will participate in the Stargate testnet as a simulated upgrade. We are anticipating the first testnet to be July 27."
A testnet is an alternative cryptocurrency blockchain, to be used for testing. Testnet coins are separate and distinct from real coins, and are not supposed to have any value. This allows application developers to experiment without resorting to using real coins and not worrying about the integrity and security of the mainnet.