Edge

A straight line connecting two vertices.

An edge connects exactly two vertices. Edges define the wireframe structure of a model and control where the surface can bend. Chains of edges form loops, and how those loops flow across a model (its edge flow) decides how cleanly it deforms and subdivides.

Related: Vertex · Face · Edge Flow · Loop Cut

These fundamentals are covered hands-on in Blender Geometry Essentials 01, free with a free account.