Relativity between two quantities (5)
I believe the answer is:
ratio
'relativity between two quantities' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ratio that I've seen before include "Comparative size" , "Mathematical relation of one thing to another" , "Eg, three to one" , "Proportion, relationship" , "Measure expressed as a proportion" .)