The word "microwave" defines itself: it means very short waves. However, what is meant by "short" depends on who is speaking and his frame of reference. Certainly ultraviolet light has a short wavelength compared to infrared, and 400 cycles per second is a higher frequency (and consequently a shorter wavelength) than 60 cycles per second. All of these waves are forms of electromagnetic energy, but none are microwaves. 

In general, radio frequencies extend from direct current up to the infrared region. The shortest wavelengths or highest frequencies of the radio spectrum are in the microwave region, but its boundaries are not clearly defined. At its high-frequency edge, it overlaps the infrared. At its low-frequency edge, technique rather than frequency is the determining factor. 

The fundamental principles underlying low-frequency radio waves and microwaves are the same. At low frequencies the observed phenomena are easily explained in terms of current flowing in a complete circuit. It is not necessary to use the idea of an electric field and a magnetic field, although these fields exist, and the observed phenomena could, in fact, be described just as well in terms of them. However, at microwave frequencies, it is usually difficult to describe the phenomena in terms of a current. 

The engineer or technician accustomed to working at low frequencies thinks in terms of "lumped" circuit elements. A radio circuit has capacitors, inductors and resistors, which are easy to locate. In a microwave circuit, the inductances and capacitances are "distributed" along a transmission line. It is impossible to point to one spot as the location of a specific circuit element. Instead, every point in the circuit is part of a distributed reactance. 

Probably the most important difference between microwave and ordinary radio techniques is the size of components relative to a wavelength. For example, in a waveguide used as a microwave transmission line, the width is greater than half a wavelength. At a frequency of 10,000 megacycles per second, where the wavelength is three centimeters, the waveguide is approximately two and a half centimeters (slightly less than an inch) wide. It is conceivable that a waveguide 90 inches wide could be used with similar results at 100 megacycles. If it were so used, it would be a microwave circuit even though it had a lower frequency than that usually referred to as a microwave. Because wavelength is not a determining factor, there is no sharp demarcation between radio waves and microwaves. 

The microwave position of the electromagnetic spectrum has been set arbitrarily between 300 megacycles (wavelength = 100 centimeters) and 300,000 megacycles per second (wavelength = 1 millimeter). The latter is the edge of the infrared region. However, as has been pointed out, this is not a rigid definition but depends on the techniques used. It is customary to omit the words "per second" from the frequency unless there is a chance of confusion.