The COLUMN keyword introduces a table column (DEFINE) in a query (also known as query column).
The syntax after COLUMN defines a query column, which has a data lineage and it is populated in an empty filter context before the query is executed.
The following example shows using a query column in a DAX query.
DEFINE COLUMN Customer[First Letter] = LEFT ( Customer[Name], 1 ) EVALUATE SUMMARIZECOLUMNS ( Customer[First Letter], "Sales Amount", [Sales Amount] ) ORDER BY Customer[First Letter]
Contributors: Alberto Ferrari, Marco Russo
Microsoft documentation: https://docs.microsoft.com/en-us/dax/define-statement-dax