Final answer:
The combining form that means tail (downward) is 'caud-,' used in anatomical terminology to describe positions near or toward the lower part of the spinal column.
Step-by-step explanation:
The combining form meaning tail (downward) is caud-. This term is often used in anatomical terms to indicate a position below or lower than another part of the body, near or toward the tail. In humans, this would refer to the area around the coccyx, or the lowest part of the spinal column. An example of this is the term caudal, which describes a position that is closer to the tail or posterior end of the body relative to other structures. Another related term is cauda equina, which refers to a bundle of spinal nerves and spinal nerve rootlets.