Oracles are data feeds that connect Ethereum to off-chain, real-world information, so you can query data in your smart contracts. For example, prediction market dapps use oracles to settle payments based on events. A prediction market may ask you to bet your ETH on the next president of the United States. They’ll use an oracle to confirm the outcome and pay out to the winners.
Oracles | ethereum.org
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Chainlink enables smart contracts on any blockchain to leverage extensive off-chain resources, such as tamper-proof price data, verifiable randomness, external APIs, and much more.