In the context of blockchain oracles, what exactly is an "event"?

An "event" is any activity that takes place on the blockchain — a transaction occurring, data being persisted, a new block being added — that an oracle's listener can subscribe to.

The word is deliberately broad. Oracles that react to the chain (pull-based inbound, push-based outbound) work by having an Event Listener subscribe to these on-chain happenings and wake up the Controller when one occurs. Concretely, on a platform like Ethereum a smart contract can emit a log entry when something noteworthy happens, and off-chain code watches for exactly those. Understanding "event" as "anything the chain did that we can be notified about" is what makes the listener-driven pattern designs click.