Ratio of AB to BC, say (4)
I believe the answer is:
sine
'ratio of ab to bc say' is the definition.
(I've seen this in another clue)
This is all the clue.
(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" .)