Triangle or quadrilateral (7)
I believe the answer is:
polygon
'triangle or quadrilateral' is the definition.
(I know that triangle is a type of polygon)
This is all the clue.
(Other definitions for polygon that I've seen before include "Many-sided plane figure" , "Plane figure with a number of straight sides" , "Geometric figure with five or more sides" , "Closed plane figure with three or more straight sides" , "Figure with many sides" .)