Question

define vertices in quadrilateral​

Answer 1

vertices are refers to the corner of any figure...........
it may be any quadrilatral, triangle, rectangle, square etc.

Answer 2

Euclidean plane geometry, a quadrilateral is a polygon with four edges (or sides) and four vertices or corners. Sometimes, the term quadrangle is used, by analogy with triangle, and sometimes tetragon for consistency with pentagon (5-sided), hexagon (6-sided) and so on.