MODELLING CONCEPTS

Face.

A collection of vertices connected in a clockwise or anticlockwise arrangement make a face.

Faces are also called polygons and the terms are completely interchangeable in 3D.

A face must have all of it's edges connected on all sides and filled in to be considered a face.

Blender allows both planar and nonp-planar faces to exist but the use of non planar faces is considered problematic as the ultimate triangulation (as happens when finalizing a version of a model for as particular destination or at render time) becomes unpredictable (although renderering engines can interpret and display them.)