In Euclidean geometry, a tangential quadrilateral (sometimes just tangent quadrilateral) or circumscribed quadrilateral is a convex quadrilateral whose sides all can be tangent to a single circle within the quadrilateral. This circle is called the incircle of the quadrilateral or its inscribed circle, its center is the "incenter" and its radius is called the "inradius". Since these quadrilaterals can be drawn surrounding or circumscribing their incircles, they have also been called "circumscribable quadrilaterals", "circumscribing quadrilaterals", and "circumscriptible quadrilaterals". Tangential quadrilaterals are a special case of tangential polygons.