VSWR WOW measurement
Given I have never been in microwave RADAR R&amp;D only VLF and VHF and I have never heard WOW,  except when someone mutters something about a VSWR of 1.2 at the transmitter and 10 at the antenna, I have a hunch.  

Also, it's not the low-frequency rumble with wow &amp; flutter.

Worst (change) over Wavelength  = WOW for millimeter wavelength microwave devices.

This parameter describes how much the VSWR changes over a full rotation at the "worst" frequency within the specified frequency range.

Think of it as the incremental impedance loss variation measured by reflection ratio or return loss over a full wave length or 360 deg.
Even the Chinese engineer in suppliers of these products understands it.