biangle

noun

1. A digon or bigon; a two-sided shape (especially in non-Euclidean geometry)“If n planes be drawn through any axis of the right-vector, each of which makes angles π/n with the planes on either side of it, the whole of space is divided into n congruent figures which may be called biangles, the space between any two adjacent planes being easily seen to be continuous with the vertically opposite space between them.”