LEGO Color Sensor - EV3
Based on this API reference and this discussion of the color sensor it looks like "pct" is short for percent as in a ratio with 100.

David added in a comment:


  EV3 hardware developer kit provides a bit more info on what 100% means: 
  
  
    The output from the sensor within light sensor mode is a value between 0 – 100 where 0 equals no reflected light measured and 100 equals full light reflection.