1 Geometry (of a line, arc, or figure) form (an angle) at a particular point when straight lines from its extremities are joined at that point:the angle subtended by a string of length r at the centre of the sphere
(of an angle or chord) have bounding lines or points that meet or coincide with those of (a line or arc).
2 Botany (of a bract) extend under (a flower) so as to support or enfold it:the main beauty is provided by the bracts which subtend the flowers
late 16th century(in sense 1): from Latin subtendere, from sub- 'under' + tendere 'stretch'. sense 2 dates from the late 19th century
subtend in other Oxford dictionaries
Definition of subtend in the US English dictionary