The DEFINE keyword is part of the EVALUATE syntax.
One DEFINE can be applied to one or more EVALUATE statements.
The DEFINE keyword can include definitions of query variables (VAR), query measures (MEASURE), query columns (COLUMN), query tables (TABLE), and functions (FUNCTION).

The following example shows a query using a query variable in two EVALUATE statements.

DEFINE
    VAR LastSalesDate =
        MAX ( Sales[Order Date] )
EVALUATE
CALCULATETABLE (
    SUMMARIZECOLUMNS (
        'Product'[Product Name],
        "Balance", CALCULATE (
            [Amount],
            'Date'[Date] = LastSalesDate
        )
    )
)

EVALUATE
CALCULATETABLE (
    SUMMARIZECOLUMNS (
        Store[Store Name],
        "Balance", CALCULATE (
            [Amount],
            'Date'[Date] = LastSalesDate
        )
    )
)