Triadic closure is a concept in social network theory, first suggested by German sociologist Georg Simmel in the early 1900s. Triadic closure is the property among three nodes A, B, and C, such that if a strong tie exists between A-B and A-C, there is a weak or strong tie between B-C. This property is too extreme to hold true across very large, complex networks, but it is a useful simplification of reality that can be used to understand and predict networks.
Read more about Triadic Closure: History, Measurements, Causes and Effects, Strong Triadic Closure Property and Local Bridges