An airliner is a great fixed-wing aircraft whose key function is for the carry of paying passenger. Airliners are typically operated by an airline company which owns or leases the aircraft.
An Airbus A340 airliner operated by Air Jamaica. The description of an airliner can differ from country to country, but typically, 20 or more traveler seats or an unfilled weight above 50,000 lb qualifies an aircraft as an airliner.
Passenger aircraft with fewer than 20 passenger places are called commuter aircraft or air taxis, depending on their size, engines and seating configurations.
While piston engines were common on prop liners like the Douglas DC-3 until the opening of the jet age, nearly all modern airliners are powered by turbine engines, either turbofans or turboprops, since they work resourcefully at much higher altitudes, are far more dependable than piston engines, and provide a ride with less shaking and noise for the passengers.
There may be variants of an airliner that are residential for moving freight or for luxury corporate use. Many airliners have also been adapted for government use as "VIP" transports and for various unarmed functions such as airborne, air ambulance, reconnaissance as well as the more obvious troop-carrying roles.