Theta-pinch, or θ-pinch, is a type of fusion power reactor design. The name refers to the configuration of magnetic fields used to confine the plasma fuel in the reactor, arranged to run around a cylinder in the direction normally denoted as theta in polar coordinate diagrams. The name was chosen to differentiate it from machines based on the pinch effect that arranged their fields running down the centre of the cylinder; these became known as z-pinch machines, referring to the Z-axis in cartesian coordinates.