Note: The VSWR is a measure of impedance mismatch between the transmission line and its load. The higher the VSWR, the greater the mismatch. The minimum VSWR, i.e., that which corresponds to a perfect impedance match, is unity.
Source: from Federal Standard 1037C and from MIL-STD-188
The VSWR is also defined as the ratio of Maximum voltage to minimum voltage in a transmission medium.
VSWR = Vmax/Vmin
Where, Vmax = Voltage level of the transmitted wave
Vmin = Voltage level of the reflected wave
If the characteristic impedance of the medium is equal to load impedance then VSWR is equals to 1.
If the characteristic impedance (Z0) is less than the load impedance (Zl) then VSWR is greater than 1.