The D820 was one of the Pentium D processors, and that series was made on a 90nm process. nm is nanometers, or one one-billionth of one meter in length. This is the width of the gates. A transistor at its most basic is a series of P and N gates that allow or restrict current based on bias, and the design process size (measured in nanometers in the last 10 or so years, microns before that) is simply the width of those gates