RF Directional Coupler - definitions
Directional coupler from here: -




  Coupling Loss - Amount of power lost to the coupled port (3) and to
  the isolated port (4). Assuming a reasonable directivity, the power
  transferred unintentionally to the isolated port will be negligible
  compared to that transferred intentionally to coupled port.
  
  Main line loss - Resistive loss due to heating (separate from coupling
  loss). This value is added to the theoretical reduction in power that
  is transferred to the coupled and isolated ports (coupling loss).
  
  Directivity - Power level difference between Port 3 and Port 4
  (related to isolation). This is a measure of how independent the
  coupled and isolated ports are. Because it is impossible to build a
  perfect coupler, there will always be some amount of unintended
  coupling between all the signal paths.
  
  Isolation - Power level difference between Port 1 and Port 4 (related
  to directivity).


So, if the coupled port (3) receives -20 dB of the power from the input port (1), the power fed to the isolated port (4) would be another 20 dB lower.