Proportional relationship (5)
I believe the answer is:
ratio
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">proportional relationship</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for ratio that I've seen before include "3 relation" , "E.g. sin i" , "Mathematical relation of one thing to another" , "Relationship between two numerical quantities" , "Relative magnitudes of two quantities" .)