Edge Flow
How edge loops travel across a model's surface.
Edge flow describes the paths that edges and edge loops trace across a mesh. Clean flow follows the natural contours of the form, which makes subdivision behave, deformation look right, and further edits easy. Poor flow shows up as pinching, shading artifacts, and fighting the mesh.
Related: Edge · Quad · Loop Cut · Pole
These fundamentals are covered hands-on in Blender Geometry Essentials 01, free with a free account.