A fleet vehicle is essentially a vehicle that a company uses during operations of a company. Fleet vehicles are owned or leased by the company. Vehicles which are used in the normal operation of a company, but are owned by company employees are not fleet vehicles.
Some examples are vehicles operated by car rental companies, taxi companies, public utilities, bus companies, and police departments and government cars.
The definition for fleet varies by state and by insurance companies. The number of vehicles owned by the business to be considered a fleet may vary. If your state does not have its own definition that governs insurers then each insurer in your may have different guidelines to determine how many cars are required to make a fleet.