Design Nomenclature / Form & Geometry
Face
Also called polygon, poly, facet
A region bounded by a closed loop of edges — the smallest unit a renderer shades. Its vertices are assumed coplanar; three always are, four or more may not be, and a non-planar face is drawn as whatever triangulation the pipeline happens to choose. 'Face' and 'polygon' are used interchangeably and mean the same thing.
In practice
'Poly count' almost always means triangle count after triangulation, not faces as authored. A quad-heavy model doubles at export — check which number a spec is asking for before quoting one.