Mathematical ratio (4)
I believe the answer is:
sine
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mathematical ratio</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 sine that I've seen before include "Opposite/hypoteneuse" , "Trigonometrical function" , "Indefinitely, with no appointed date - . . . . die" , "Mathematical function" , "Latin tag for indefinitely - . . . . die" .)