An expression is a combination of fields, variables, operators, functions, numbers, and mathematical symbols put together according to a special syntax in order to calculate a value. Expressions are used both in scripts and in chart visualizations. They can be simple, involving only basic calculations, or complex, involving functions fields and operators.
In a script, an expression is evaluated as the script execution passes it by. In visualizations (including charts and tables), expressions are evaluated automatically whenever any of the fields, variables or functions that the expression contains change value or logical status.
Expressions can be used in several different situations. The difference between measures and expressions is that expressions have no name or descriptive data.