Pole
A vertex where three, five, or more edges meet instead of four.
In an all-quad mesh most vertices join four edges. A pole is a vertex joining three, five, or more. Poles are unavoidable and useful for redirecting edge flow, but placed badly (on a curved, deforming, or highly visible surface) they cause pinching and shading artifacts.
These fundamentals are covered hands-on in Blender Geometry Essentials 01, free with a free account.