Vertex

Note:For the Hammer Vertex Tool, click here.

A vertex is a point in space that forms one of the three points of a  polygon. It can be represented with an XYZ vector, such as (0 0 0).

Vertices (not "vertexes"!) are by themselves invisible. It is the polygon they come together to form that is rendered. However, they do contain a 'normal' component that decides the direction from which the most light is received by the attached polygon corner. This is also represented by a vector: the default is (0 0 1), which means "full intensity straight up".