Final answer:
Pitch diameter is the diameter of the pitch circle, an imaginary circle that passes through the threads where the width of threads and gaps are equal. It's crucial in mating threaded parts and is different from thread pitch, or distances from crests to roots or between major and minor diameters.
Step-by-step explanation:
The term pitch diameter is a concept often used in engineering and manufacturing, particularly when discussing screw threads. It refers to the diameter of the pitch circle in a helical or threaded part. The pitch circle is an imaginary circle that passes through the profile of the threads at a point where the width of the threads and the spaces between the threads are equal. To put it simply, the pitch diameter is the diameter of an imaginary co-axial cylinder that intersects the thread profile at these points, and it is critical when mating two threaded parts together.
In the context of screws and threaded fasteners, the pitch diameter is not the same as the distance between two adjacent threads (often referred to as the thread pitch), nor is it the distance from the crest (top) to the root (bottom) of a thread, or the distance between the major (outermost) and minor (innermost) diameters of a thread. Therefore, the correct answer to the student's question is (1) The diameter of the pitch circle.