line at infinity
英[lain æt ɪnˈfɪnɪti:]美[laɪn æt ɪnˈfɪnɪti]
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.