Proportional relation (5)
I believe the answer is:
ratio
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">proportional relation</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)