Apeirogon

An apeirogon is a degenerate polygon with a countably infinite number of sides.

Like any polygon, it is a sequence of line segments (edges) and angles (corners). But whereas an ordinary polygon has no ends because it is a closed circuit, an apeirogon can also have no ends because you can never make the infinite number of steps needed to get to the end in either direction. Closed apeirogons also exist. They occur when the corners form sequences (one in each direction, starting from any point) whose limits converge on the same point. Such a point is called an accumulation point, and any closed apeirogon must have at least one of them.

Two apeirogons can tessellate the plane, and the Schläfli symbol for this tessellation is {∞, 2}.