In an adult, what does 75 to 90 mm of mercury denote?

The blood pressure of an individual is measured in mercury.

In a healthy man, systolic pressure (the amount of pressure experienced by the arteries while the heart is beating) is 120 mm Hg, and diastolic pressure (the pressure exerted on the walls of the arteries when the heart muscles relax in between two beats) is 80 mm Hg. 

Thus, 75 to 90 mm of mercury in an adult denotes diastolic pressure.