The official definition of a "parked car" can differ depending on the insurance company or state laws.
In general, a car is defined as parked if it is stopped and unattended off of a public roadway. Sometimes a parked car can also be defined as a vehicle that is stopped, occupied and running, but out of the roadway or in a parking lot or facility.
Typically, a car at a red light is considered stopped and not parked. Normally a vehicle is still in gear and on a public roadway when at a light or stop sign so therefore is not parked. A vehicle that is occupied with the engine running might be considered parked if the car is still in "park" as the gear selected and is off of the roadway in a parking spot.