What is the term used to define the bottom surface that joins two sides of a thread in a screw?

The correct term that defines the bottom surface joining two sides of a thread in a screw is referred to as the root. In the context of screw threads, the root is the portion of the thread that is located at the base, where the thread ridges or flanks converge. It represents the minimum diameter of the screw's thread profile.

Understanding the root is critical for various applications in machine design, particularly when it comes to ensuring that a screw can effectively engage with a nut or a tapped hole, providing strength and reliability in fastening applications. The geometry of the root can influence the strength and load-carrying capacity of the screw, making it a vital aspect of thread design.

In contrast, the other terms mentioned do not accurately describe this specific feature. The pitch refers to the distance between corresponding points on adjacent threads, the crest is the top surface of the thread where it peaks, and the term "bottom land" is not a standard term used in thread nomenclature. Thus, the understanding of the root and its significance in screw design is essential for engineers and designers to create effective mechanical components.