Screw thread

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.