What is the definition of 'arm' in aviation terminology?

In aviation terminology, the term 'arm' refers to the horizontal distance from a specific reference point known as the datum. The datum is an imaginary vertical plane that serves as a reference for measuring various distances on the aircraft. This measurement is crucial because it helps in calculating the moment, which is the product of the weight of the aircraft component and its arm. Understanding the arm is essential for weight and balance calculations, ensuring that the aircraft is loaded correctly and can maintain proper flight stability.

The other options do not accurately capture the meaning of 'arm' as it pertains to aviation. While the distance from the wing to the tail and the vertical distance from the ground might be important factors for other aspects of aircraft design, they do not pertain to the concept of arm. Similarly, the length of the aircraft's body is a different measurement altogether, unrelated to the arm in the context of weight and balance.