Proportional relationship (5)
I believe the answer is:
ratio
'proportional relationship' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ratio that I've seen before include "3 relation" , "E.g. sin i" , "Mathematical relation of one thing to another" , "Relationship between two numerical quantities" , "Relative magnitudes of two quantities" .)