Relative magnitudes of two quantities (5)
I believe the answer is:
ratio
'relative magnitudes of two quantities' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ratio that I've seen before include "nine-to-five, say" , "Proportion, relationship" , "Proportional relation between two numbers" , "Quotient of two numbers" , "Proporation, quantitative relation" .)