Note that there is no single agreed definition of a Blockchain Oracle. Here are a few selected definitions I came across.
An oracle is an agent that finds and verifies real-world occurrences and submits this information to a blockchain to be used by smart contracts.
"Oracles are trusted data feeds that send information into the Smart Contract, removing the need for Smart Contracts to directly access information outside their network, thus lightening their workload. Oracles are usually supplied by third parties and are authorized by the companies that use them.” - https://medium.com/bethereum/how-oracles-connect-smart-contracts-to-the-real-world-a56d3ed6a507
"Oracles are network services that, upon request, provide commands that encapsulate a specific fact (e.g. the exchange rate at time x) and list the oracle as a required signer.” - https://docs.corda.net/key-concepts-oracles.html
"An oracle is any mechanism that connects real-world data to a blockchain. It can be an API provided by a stock exchange, for example, or, in the case of a smart energy grid, it can be a metering device.” - https://hackernoon.com/the-oracle-of-truth-where-do-blockchain-betting-projects-get-their-event-results-r44b2c99
"In blockchain, oracle refers to the third-party or decentralized data feed services that provide external data. Oracle provides interfaces from the real world to the digital world.” - https://hackernoon.com/overview-of-advance-ethereum-concepts-of-oracle-off-the-chain-data-pos-and-tps-e0sk2dc4
"an oracle provides a connection between real world events and a blockchain” - https://medium.com/hackernoon/oracles-help-smart-contracts-resolve-subjective-events-d81639d8291c
Feeling lost? you might want to start reading here Impact Oracles category: START HERE
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