Three different ways to notate a generalized impedance.

Top is the same symbol as a resistor, USA style.
Middle is same symbol as a resistor, Euro style.  (Sedra/Smith uses this to notate a generalized impedance while using the squiggle style to notate a resistance. Also [1][2])
Bottom is a labeled box.  (Carlson uses this to notate a generalized impedance while using the squiggle style to notate a resistance.  Also [3][4])
This W3C-invalid circuit diagram was created with the Electrical Symbols Library.
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