Mathematical proportion (5)
I believe the answer is:
ratio
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mathematical proportion</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for ratio that I've seen before include "Relationship between two numerical quantities" , "Relative magnitude" , "Relativity between two quantities" , "Eg, three to one" , "Expression of relationship" .)