Point in space, usually a point in a mesh
model. A vertex can be both a 3D or 2D point. Not to confuse with vector
allthough conversion can be done. A vertex usually contains more than just a position. It can also contain color, mapping coordinates
and other properties.
One vertex - Many vertices.
Typical vertex definition in C:
Three or more vertices makes a polygon, if connected.
This model has over 100k vertices. Transforming each vertex is nothing you do by hand!