Semantic Layer & Headless BI tools establish a centralized business logic layer between data warehouses and consumption endpoints, ensuring consistent metric definitions across dashboards, embedded analytics, applications, and AI agents. These platforms define metrics, dimensions, and relationships once in code, then expose them through APIs rather than requiring users to interact with a specific BI interface. The category spans pure-play semantic layers focused exclusively on metric governance and API-first delivery, hybrid platforms combining semantic modeling with OLAP or caching capabilities, and headless BI systems that separate query engines from presentation layers. Originally dominated by enterprise OLAP cubes and proprietary BI server architectures, the category has evolved toward cloud-native, version-controlled semantic layers that treat metrics as code and integrate with modern data stacks through SQL and REST APIs.