Define subsonic flight.

Subsonic flight refers to speeds at which airflow is below the speed of sound. This is characterized by speeds that are generally less than Mach 1, where Mach 1 is the speed of sound in air at sea level, approximately 343 meters per second or 1,125 feet per second. When an aircraft is flying in subsonic conditions, the airflow around it is stable and the shock waves associated with supersonic speeds are not present. Understanding subsonic flight is essential as it impacts aerodynamics, fuel efficiency, and flight design.

In contrast, other definitions provided in the options refer to speeds that do not align with the description of subsonic flight. Speeds above the speed of sound denote supersonic flight, while any mention of Mach 2 indicates hypersonic speeds. Similarly, speeds equal to Mach 1 perfectly describe transonic flight, not subsonic. Recognizing the specific characteristics and definitions in relation to speed is important for anyone studying aerodynamics or aviation.