Why is DC Voltage called Direct Current ?
"DC Voltage" and "AC Voltage" are gramatically incorrect, but everyone uses those terms.

More gramatically correct terms if we are talking about voltage would be "DV (Direct Voltage)" or "AV (Alternating Voltage)" - but no-one says that.

DC Voltage is a voltage associated with an unchanging current, while AC Voltage is the voltage associated with a current that is continually changing direction.