Range of motion
Range of motion refers to the ability to move and the extent of movements in taijiquan. It is NOT the same as flexibility.
In taijiquan, the primary action is a rotation. In order to rotate using the human body, body parts must take actions to allow or facilitate a rotation. These actions include stretching. Stretching includes range of motion. This line of thinking will lead to a particular type of action, unlike flexibility, though there are similarities.