Final answer:
The term that best describes how far one's arm can move backward in preparation for a tennis forehand swing is 'range of motion.' This term reflects the joint's full movement potential, specifically the shoulder's flexibility allowing for the necessary backswing.
Step-by-step explanation:
The term most representative of determining how far one's arm can move backward in preparation for a forehand swing in tennis is range of motion. Range of motion is the full movement potential of a joint, usually its range of flexion and extension. In terms of a forehand swing in tennis, the athlete relies on the flexibility of the shoulder to achieve the necessary backswing, which encompasses the range of motion. Flexibility is a component of range of motion, but it specifically refers to the ability of muscles to lengthen to allow movement at the joint. While strength and agility are important for power and quick adaptive movements, respectively, they do not primarily describe the extent to which the arm can move.