A two-dimensional representation formed by perpendicular intersections of lines drawn from points on the object being pictured to a plane of projection.

1. A continuous linear map P of a Hilbert space H onto a subspace M such that if h is any vector in H, h = Ph + w, where w is in the orthogonal complement of M.

2. A mapping of a configuration into a line or plane that associates to any point of the configuration the intersection with the line or plane of the line passing through the point and perpendicular to the line or plane.