What characterizes tricycle landing gear?

Tricycle landing gear is characterized by the configuration that includes a third wheel located under the nose of the aircraft. This design typically consists of two main wheels positioned under the fuselage and a smaller nose wheel at the front, which helps in maintaining stability during ground operations. The arrangement allows for better balance and control, especially during landing and takeoff phases, as the nose wheel assists in steering the aircraft while taxiing. This design is prevalent in modern aircraft because it minimizes the risk of a nose-over incident and provides greater visibility for the pilot during ground maneuvers.

The other configurations mentioned do not represent a tricycle design; for example, a single wheel under the tail relates to taildragger setups, while having two wheels under each wing pertains to designs where gear configurations may not offer the same ground handling advantages provided by the tricycle arrangement. Retractable wheels are a feature that can be present in various gear types but is not a defining characteristic of tricycle landing gear specifically.