A synthetic jet is a zero mass flux device that converts electrical energy into momentum. These devices are being explored for use in several different fields, including aerospace and electronics.
A typical synthetic jet consists of a piezoelectric element at one side of a small chamber opposed by a small orifice, usually a hole or slit. Current synthetic jets are on the scale of one to two inches (two to four centimeters) in diameter, with a slit of similar size (or a pinhole). Applying voltage to the piezoelectric element causes it to vibrate, and outside air is rapidly pulled into the chamber through the orifice then expelled. The synthetic jet is referred to as a "zero mass flux" device because the net mass flowing from the jet to the outside air is zero. The mass of the air drawn in is equal to the mass of the air expelled.
An example: Imagine sitting on a frictionless surface (fun, isn't it?) Make an "O" shape with your mouth, then breathe in. You don't move. Breathe out through your mouth, still making an "O" shape, and you will move opposite the direction you are breathing, per Newton's Third Law of Motion. Repeat this process several thousand times per second, and you will be a synthetic jet.
By placing synthetic jets on the skin of an aircraft, the flow around the aircraft can be controlled. This can improve the aircraft's efficiency through boundary layer control or even induce a control moment, making the aircraft pitch, roll, or yaw. These devices might also be used to cool electronics such as CPUs. Synthetic jets are not being used in any current products but applied research has been done for about a decade and continues today.