DAX (Data Analysis Expressions) is a functional formula language developed by Microsoft for data modeling and analytical calculations in Power BI, Excel Power Pivot, and SQL Server Analysis Services. Unlike traditional spreadsheet formulas, DAX operates on tables and columns rather than cell references, enabling dynamic aggregations that automatically respond to filter context. The key to DAX mastery lies in understanding evaluation contexts: filter context (what's being filtered), row context (iterating through rows), and context transition (how CALCULATE bridges the two) — once you grasp these three concepts, even complex DAX patterns become intuitive.