Defeaturing is a method of simplifying a mesh by removing protrusions and holes. This can increase rendering performance by decreasing the number of vertices and triangles the mesh contains.
Defeaturing can be particularly useful for geometry that comes from computer-aided design (CAD) applications. When parts and assemblies are designed and modeled for the purpose of physical manufacturing, they are often created with small features that are essential for the real-world objects being manufactured, but not necessary for real-time rendering.
For example, this part from a fuel pump assembly contains 20,000 triangles. Removing some of the holes cuts that number almost in half. In this case, other parts in the assembly cover the modified areas, so the rendered result does not change noticeably when seen in context.