Sentence:  They [the students] also came upon new and unusual mathematical figures: the <WSD>digon</WSD>, a two-sided polygon on a spherical space, and the apeirogon, an open polygon with infinitely many sides nb.... All these discoveries brought up even more questions. Is a circle a polygon? What makes an octagon an octagon – its eight vertices, its eight sides, or both? Can a polygon cross itself? Does a polygon need to be closed? 
Meaning of the word marked with <WSD> tags:  A polygon having two edges and two vertices.