Final answer:
The visible joint between the upper and lower parts of the arm is the elbow joint, formed by the humerus, radius, and ulna bones.
Step-by-step explanation:
The visible joint between the upper and lower parts of the arm is known as the elbow joint. This joint is formed by the articulation of the humerus bone of the upper arm with the radius and ulna bones of the forearm. Specifically, the trochlea of the humerus fits into the trochlear notch of the ulna, while the capitulum of the humerus articulates with the head of the radius, allowing for the flexion and extension of the arm.