The term "semantic layer" has been part of the data industry's vocabulary for over 35 years. It first appeared in a 1991 patent filing by Business Objects, and it has since been reinvented, abandoned, and reinvented again across three distinct eras of data architecture. Today, it sits at the center of one of the most consequential design debates in the industry: should the semantic layer be a standalone product you bolt onto your stack, or a native capability of the platform that already manages your data? This guide covers the full arc: what a semantic layer is, where it came from, how it split into two competing architectural approaches, and why the choice between them determines whether your AI agents produce accurate answers or hallucinated nonsense.…