Ratio of AB to BC, say (4)
I believe the answer is:
sine
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">ratio of ab to bc say</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 sine that I've seen before include "Mathematical function" , "Trig function" , "Trigonometric function" , "Latin tag for indefinitely - . . . . die" , "Indefinitely, with no appointed date - . . . . die" .)