What is a w/t ratio? I have seen a few guides mentioning w/t ratio but I can't find anything which defines it.  

I am using Simplify3D with a Wanhao i3.
It appears to refer to calibration factor called Width over Thickness. 

From Calibrate your 3D printer to print parts to fit


  Here is the list of necessary variables for this calibration to work,
  
  
  Carve/Extra Decimal Places (float) : change to 5
  Carve/Edge Width over Height (ratio) : nozzle diameter/layer height
  Inset/Infill Width over Thickness (ratio) : nozzle diameter/layer height (initially)
  Dimension/Filament Packing Density (ratio) : needs calibration (equivalent to the reciprocal of the extrusion multiplier in Slic3r)
  Scale/XY Plane Scale (ratio) : needs calibration