An airliner is a large, commercial aircraft operated by an airline for transporting people and/or cargo.
There's no formal, precise definition of an "airliner" but it's usually understood to mean any large aircraft used by a scheduled airline to carry people and/or cargo.
An airliner is identified mainly by size: few people would call a Cessna Caravan an airliner even though it's used in scheduled airline services in various parts of the world. Airliners may have any type of engine: (piston) propeller, turboprop or turbofan.
Use this tag for general questions about aircraft used for commercial airline operations, but if the question is about a specific make or model of aircraft then it's better to use a more specific tag, e.g. boeing or airbus-a320.
- Airliner (Wikipedia)