SWR may be defined as ratio of max to min current on voltage on a line having standing waves. 
 VSWR=Vmax/Vmin
 S=Vmax/Vmin=Imax/Imin
 The SWR is a measure of mismatch between load of transmission line and is first and foremost quantity calculated for a particular load. Its value is always greater than unity when termination is not correct. But when termination is correct, its value is equal to unity, if termination is perfectly matched. 