Rigor is something strict, severe or demanding. 1  Harsh and strict treatment in the military for cadets is an example of rigor. 2  Freezing weather and ice are examples of the rigors of winter.  Difficult and challenging academic courses are an example of academic rigor.
The suffix gram means to draw or write. This is used in telegram.
A gram (g) is the basic unit of measurement of mass used in the metric system. Nanograms (ng) and milligrams (mg) are both units of grams. Nano means one-billionth. Therefore, a nanogram is a billionth of a gram. Milli means one-thousandth. Therefore a milligram is a thousandth of a gram. One milligram is equal to one million nanograms.