Technically it is exactly medium because most people use it as the standard for measuring density.
It's relatively high compared to something with a lower density... such as air.... yet, it's density is relatively low compared to something with a higher density... like iron.
It really depends on the temperature, Water vapor is low, ice is high, liquid in between.
In comparison to what?
1g/cm3<br /><br />
Are you designing an atom bomb?