GND, 0 Volt, 0-potential, abbreviation for “Ground”. The voltage or “potential difference” is measured against this potential. The voltage potential of positive voltages is higher than GND, negative voltages have a voltage potential below GND. In colloquial terms, GND is often mistakenly referred to as negative supply voltage. A current (positive or negative) flows back to the voltage source via the GND line.
The problem with this (or every) naming is that these are only names, not binding standards or norms. Therefore, great care should always be taken when assigning such names in the circuit diagram design, and only introduce new or additional names if the supply voltage in question is physically decoupled from other voltages in the circuit (e. g. via a choke coil), and if it is used on several component pins.