Proportional relation (5)
I believe the answer is:
ratio
'proportional relation' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ratio that I've seen before include "Relative magnitude" , "Proportion between two numbers" , "Relative magnitudes of two quantities" , "Measure expressed as a proportion" , "five to four, perhaps" .)