dbt Cloud Semantic Layer is a declarative way to define business metrics, dimensions, and relationships. Once defined in dbt, metrics can be queried from any BI tool (Looker, Tableau, Metabase). Prevents duplicating metric definitions across tools and ensures consistency. Example: Define revenue = sum(amount) by date once in dbt → Looker and Tableau both query revenue metric from dbt → Both get same definition, guaranteed consistent.