In
Euclidean geometry, a
tangential quadrilateral (sometimes just
tangent quadrilateral) or
circumscribed quadrilateral is a
convex quadrilateral whose sides are all
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.