What is Vee on a 74hc4051 mux?
The 74HC4051 has level shifters inside that allow you to use a logic supply that is GND to Vcc and another supply voltage Vee that is less than (or equal to) GND. 

This is a great advantage when you want to use dual supplies such as +/-5V for the analog electronics, and the digital signals are 0 to +5 (or +/-3.3V with digital logic 0/3.3V), and if you don't need the feature you can just ground Vee. 

I am not sure I believe the operating area shown here:  



The early CD4051 clearly required Vee &lt;= GND.