Normal (geometry) Normal (geometry) In geometry, a normal is an object such as a line or vector that is perpendicular to a given object. For example, in two dimensions, the normal line to a curve at a given point is the line perpendicular to the tangent line to the curve at the point. In three dimensions, a surface normal, or simply normal, to a surface at a point "P" is a vector that is perpendicular to the tangent plane to that surface at "P". The word "normal" is also used as an adjective: a line normal to a plane, the normal component
Incidence (geometry) Incidence (geometry) In geometry, an incidence relation is a heterogeneous relation that captures the idea being expressed when phrases such as "a point "lies on" a line" or "a line is "contained in" a plane" are used. The most basic incidence relation is that between a point, , and a line, , sometimes denoted . If the pair is called a "flag". There are many expressions used in common language to describe incidence (for example, a line "passes through" a point, a point "lies in" a plane, etc.) but the term "incidence" is preferred because it does not have the