A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread.

The most frequently used thread form is the 60° thread form found in the metric ISO thread and in the UN-series. However, there are other thread standards as well. These are either based on foreign standards or are used for special applications. The most common threads are:

• ISO Thread (metric)

• UN (Unified National) series Thread

• Pipe Thread

• Whitworth Thread

• Trapezoidal Thread

• Knuckle Thread

• Buttress Thread
thread

Major diameter: The diameter of a screw thread measured across the crest of the threads and perpendicular to the thread axis.

Pitch diameter: The diameter on a threaded object that lies between the major and minor diameters.

Minor diameter: The diameter of a screw thread measured across the thread root of the threads and perpendicular to the thread axis. 

Pitch: The distance from crest to crest.