Final answer:
The post that supports handrails at the end of a stairway is known as a newel post. It is a structural and often decorative element of stair design.
Step-by-step explanation:
On an interior stair system, the post that supports the handrails at its termination is called a newel post. This structural element is often a larger, more decorative post and serves not only as a support for the handrail but can also be a key design feature in the overall aesthetic of the staircase.
The newel post can be found at the bottom and top of the staircase, and sometimes at key transitional points such as landings or changes in direction.