Microwaves, a subset of radio waves, have wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

s ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 between 300 MHz (0.3 GHz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

) and 300 GHz. This broad definition includes both UHF and EHF
Extremely high frequency
Extremely high frequency is the highest radio frequency band. EHF runs the range of frequencies from 30 to 300 gigahertz, above which electromagnetic radiation is considered to be low infrared light, also referred to as terahertz radiation...

 (millimeter waves), and various sources use different boundaries. In all cases, microwave includes the entire SHF
Super high frequency
Super high frequency refers to radio frequencies in the range of 3 GHz and 30 GHz. This band of frequencies is also known as the centimetre band or centimetre wave as the wavelengths range from ten to one centimetres....

 band (3 to 30 GHz, or 10 to 1 cm) at minimum, with RF engineering
RF engineering
Radio frequency engineering is a subset of electrical engineering that deals with devices that are designed to operate in the Radio Frequency spectrum...

 often putting the lower boundary at 1 GHz (30 cm), and the upper around 100 GHz (3 mm).

Apparatus and techniques may be described qualitatively as "microwave" when the wavelengths of signals are roughly the same as the dimensions of the equipment, so that lumped-element circuit theory
Lumped element model
The lumped element model simplifies the description of the behaviour of spatially distributed physical systems into a topology consisting of discrete entities that approximate the behaviour of the distributed system under certain assumptions...

 is inaccurate.