在几何与拓扑学里，无穷远线（line at infinity）是一条附加于实（仿射）平面的投影线，以形成一个投影平面。加上无穷远线后，投影平面上的重合性质才具有封闭性，而没有例外。无穷远线亦被称为理想线。
英语百科
Line at infinity 无穷远线
In geometry and topology, the line at infinity is a projective line that is added to the real (affine) plane in order to give closure to, and remove the exceptional cases from, the incidence properties of the resulting projective plane. The line at infinity is also called the ideal line.