Vacuum is the full or partial elimination of Atmospheric Pressure
Atmospheric Pressure on the Moon = 0 = Full Vacuum
A system is said to be at partial vacuum if it is at a negative pressure. When you use a straw, the partial vacuum created inside the straw is what causes the liquid to travel up straw and into your mouth. A perfect vacuum would be a space with nothing in it.
1 Inch Hg Vacuum = 1.13 Ft of Water