Concave quadrilateral

…The sum of the four interior angles of a quadrilateral is four right angles. A quadrilateral in which none of the interior angles exceed two right angles is called a convex quadrilateral (Figure 1), and a quadrilateral in which none of the interior angles exceed two right angles is called a concave quadrilateral (Figure 2). In the quadrilateral ABCD, ∠A and ∠C, ∠B and ∠D are called diagonals, and sides AB and CD, and sides BC and DA are called opposite sides. …

*Some of the terminology explanations that mention "concave quadrilateral" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…四辺形の4内角の和は4直角である。どの内角も2直角を超えない四辺形を凸四辺形といい(図1)，そうでない四辺形を凹四辺形という(図2)。四辺形ABCDにおいて，∠Aと∠C，∠Bと∠Dを対角といい，辺ABと辺CD，辺BCと辺DAを対辺という。…

※「凹四辺形」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報