The term 'Plane Domain' is composed of two words: AIR (3 letters) and SKY (3 letters). AIR is defined as the invisible gaseous substance surrounding the earth, a mixture mainly of oxygen and nitrogen. SKY is defined as the space or region above the earth's surface, where clouds, birds, and airplanes can be seen. Together, these two words form the term 'Plane Domain, ' which refers to the area of the atmosphere where planes fly. This domain is typically located between 18, 000 and 45, 000 feet above the ground.