A concave quadrilateral. An arrowhead or a delta, for example.
http://wiki.answers.com/Q/What_is_non_convex_quadr...

My favorite teacher taught me what a quadrilateral is. It is a four sided polygon. It must have four corners, sides, and the interior angles need to add up to 360 degrees. For more
http://www.ask.com/webanswers/Science/Mathematics...

An isosceles trapezoid.
http://answers.yahoo.com/question/index?qid=200901...

A convex quadrilateral interior angle atsup to 4 degrees. Thanks fo...
http://www.chacha.com/question/whatdoesaconvex...
