The Rendering Pipeline - Introduction to 3D Game Programming with DirectX 9.0 - page 60

2.1 Model Representation
A scene is a collection of objects or models. An object is represented as
a triangle mesh approximation, as Figure 2.2 illustrates. The triangles of
the mesh are the building blocks of the object that we are modeling. We
use the following terms all interchangeably to refer to the triangles of a
mesh: polygons, primitives and mesh geometry. (Although triangles are
primitives, Direct3D also supports line and point primitives. However,
since lines and points aren't useful for modeling 3D solid objects, we
omit a discussion of these primitive types. We do discuss some applica-
tions of points in Chapter 14.)
Figure 2.2: A terrain approximated
by triangles
The point where two edges on a polygon meet is a vertex . To describe a
triangle we specify the three point locations that correspond to the
three vertices of the triangle (see Figure 2.3). Then to describe an
object, we specify the triangles that make it up.
Figure 2.3: A triangle defined by its
three vertices