This document provides an overview of Open RAN terminology including virtualization, disaggregation, and decomposition. It explains the evolution from traditional integrated RAN architectures to disaggregated architectures with open interfaces. The key aspects covered are horizontal openness through open interfaces, vertical openness through virtualization and compute disaggregation, and the decomposition of RAN functions into distributed units and centralized units. References for further reading on Open RAN, O-RAN, and RAN disaggregation are also provided.