Sentence:  To unify the treatment of concurrent and parallel lines, define a <WSD>point at infinity</WSD> for each family of parallel lines, and declare each <WSD>point at infinity</WSD> to lie on each of the parallel lines that define it....Now parallel lines meet at infinity, and vanishing points are the images of <WSD>points at infinity</WSD>. 
Meaning of the word marked with <WSD> tags:  (Euclidean projective geometry) Any point added to a space to achieve projective completion.