# Translations:Digital filtering/15/en

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What is the symbol *Z*? The symbol *Z* used here has a special mathematical meaning: *Z* represents an operator that produces a unit delay. Thus, we call *Z* the unit-delay operator. It follows that represents an operator that produces a unit advance. As we will see, the symbol *Z* is the variable that defines the *Z*-transform. Used in this way, *Z* is a complex variable in the complex *Z*-plane.