To add to all the good answers already provided, Microsoft defines DAX and Power Query M as two distinct 'formula languages' with different use cases (as the other answers provide in some detail). It just happens that Power BI (and Excel) combines both of these tools.
DAX is used in Power BI, Analysis Sevices and Excel Power Pivot.
Power Query M language is used in several Microsoft products, including Power BI, Excel, Power BI Report Server, Power Query Online, Power Apps, Azure Data Factory & Fabric, and Dynamics 365.
The links below provide a good coverage at different levels (overview and deep dives):