This definition comes from the technical glossary.
A global testing environment in which developers can obtain and spend satoshis that have no real-world value on a network that is very similar to the Bitcoin mainnet.
Testnet mode — Bitcoin.org Developer Examples
Testnet — Bitcoin Wiki
Get testnet bitcoins — Testnet Faucet
Bitcoin.org is community supported: 1GwV7fPX97hmavc6iNrUZUogmjpLPrPFoE