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definition. formal variable, formal expression [ag-0009]

A formal variable is an arbitrary symbol that is used to represent some mathematical object, and assumes nothing about the value or nature of the object.

A formal expression is mathematical expression with formal variables, assuming nothing but the formal variables participate in the expression have operations that are used in the expression.

A formal expression can be evaluated by replacing the formal variables with actual mathematical objects that have the operations defined in the expression.

When there is no ambiguity, we may omit the word "formal" and simply say variables or an expression.