also: にかっけい
noun
digon
A polygon with two sides and two vertices; in Euclidean geometry this is degenerate, but it exists in spherical geometry.
球面上では二角形を定義できる。
On a spherical surface, a digon can be defined.
二角形はユークリッド幾何学では退化した多角形とみなさ
A digon is considered a degenerate polygon in Euclidean geometry.
Compound of 二 (two) + 角 (angle) + 形 (shape). A straightforward Sino-Japanese coinage for the geometric concept of a digon.