Needle valves are devices that regulate fluid flow in response to a mechanical input (typically generated from an electric input signal). The control action is accomplished by changing the cross sectional area available to the flow.

The component responsible for this variation is an axially moveable body with a tapered end that occupies the gap in the entry into exit channel. The precision of this control improves when the taper angle is minimal.

Furthermore, most applications have traditionally employed a tiny moveable body, and the resulting form resembling a sewing needle has resulted in the widely used term.