The standard unit of mass is defined to be "equal to the mass" of a cyliner of platinum and iridium in france. I have always wondered what this means? Why are all sources saying mass is defined in terms of mass? What were the steps taken to decide this unit? I know that base quantities have to be defined some way at least, but in particular I am not able to get my head around this definition, I understand the ones for distance and other units. I know from mechanics tuahgt in school that mass is a measure of an objects inertia (and inertia is an inherent property in all objects). So, just like we cannot define words using the same word it confuses me when people say the standard unit of mass is the mass of the cylinder in France. What does the bolded part actually refer to?