What exactly is a diverted flight?

A diverted flight is a flight which is operated from the scheduled point of origin to a point other than the scheduled point of destination in the airline’s published schedule.

Here is an example: a carrier has a published schedule for a flight from point A to B. If the carrier were to fly an A to C operation, the A to B segment is a diverted flight.

Let’s be more precise: You book a flight from Frankfurt, Germany to Miami, FL, but your flight actually lands in Paris.