Math proportion (5)
I believe the answer is:
ratio
'math proportion' 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 "Proportion of one thing to another" , "2:1, for example" , "Relative magnitudes of two quantities" , "9 to 5, say?" , "five to four, perhaps" .)