Sentence:  In the geometry of inversion, therefore, it is usual to sacrifice the line of <WSD>points at infinity</WSD> which is so useful in other fields, and adopt the convention of a single <WSD>point at infinity</WSD>, the inverse of the center of the circle of inversion. 
Meaning of the word marked with <WSD> tags:  (Euclidean projective geometry) Any point added to a space to achieve projective completion.