N-gon
A face with more than four vertices.
An n-gon is any face with more than four points. On perfectly flat surfaces they are harmless, but on anything curved or deforming they shade unpredictably and break subdivision. Most workflows clean n-gons into quads and tris before an asset ships.
Related: Quad · Tri (Triangle) · Face
These fundamentals are covered hands-on in Blender Geometry Essentials 01, free with a free account.