From IVoM: 

ratio of two measurement units for quantities of the same kind

EXAMPLE
     km/m = 1000 and thus 1 km = 1000 m

NOTE – The measurement units may belong to different systems of units.

     EXAMPLES
     a) h/s = 3600 and thus 1 h = 3600 s;
          (km/h)/(m/s) = (1/3.6) and thus 1 km/h = (1/3.6) m/s.
     b) (km/h)/(m/s) = (1/3.6) and thus 1 km/h = (1/3.6) m/s.