A space in which there is a low pressure of gas, i.e. relatively few atoms or molecules. A perfect vacuum would contain no atoms or molecules, but this is unobtainable as all the materials that surround such a space have a finite vapour pressure. In a soft (or low) vacuum the pressure is reduced to about 10−2 pascal, whereas a hard (or high) vacuum has a pressure of 10−2−10−7 pascal. A pressure below 10−7 pascal is known as an ultrahigh vacuum (UHV). See also vacuum pump.