What is a blockchain oracle?

A blockchain oracle is a service that feeds outside data, like prices, weather, or sports scores, into a blockchain so smart contracts can use it. Blockchains can't fetch information on their own, so contracts that depend on real-world facts need an oracle to bring that data in reliably. This matters because a lot of DeFi runs on data accuracy: a lending app needs to know ETH's current price to decide when to liquidate a loan. Chainlink is the best-known example, pulling price feeds from multiple independent sources and averaging them so no single bad data point can trigger a wrong liquidation. Oracles are also a weak point: if the data feed gets manipulated, attackers can drain funds, which is why many protocols use several oracles and check for outliers before trusting a number.