I'm curious to know why 'Z' was chosen to represent high impedance state for types such as used in VHDL and VHDL-AMS's std_logic_1164 logic system:
(image of page 789 of The System Designer's Guide to VHDL-AMS: Analog, Mixed-Signal, and Mixed-Technology Modeling (Systems on Silicon) ISBN: 1-55860-749-8)
"high-Z", "hi-Z" etc. seem to be in common use, but my Google Fu is not up to the task to identify the history. The three state logic wikipedia page does not clarify it, nor does the Use of 'Z' section of the IEEE_1164 wikipedia page indicate the history.
Best Answer
'Z' is the electrical symbol for impedance.
(See https://en.wikipedia.org/wiki/Electrical_impedance)
Digital electronics has been using the terms like 'high Z' and 'hi-Z' for decades to mean 'in high-impedance state' or 'in tri-state'. It pre-dates VHDL.
So it's a logical step for 'Z' to be used as the single character notation for std_logic etc. for a signal in its high impedance state.