A screw thread, often shortened to thread, is a helical structure. It 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 tightening of a nut to a bolt is like driving a wedge into a gap until it sticks through friction and plastic deformation (how the surfaces of the two objects slightly squash together).

A screw thread is one of the six simple machines which give mechanical advantage.

The mechanical advantage of a screw thread depends on its lead, which is the linear distance the screw travels in one turn or revolution.^[1]