Mole (unit)
A mole is simply a unit of the number of things. Other common units include a dozen, meaning  12, and a score, meaning 20. Similarly, a mole refers to a specific quantity-- its distinguishing feature is that its number is far larger than other common units. Such units are typically invented when existing units can not describe something easily enough. Chemical reactions typically take place between molecules of varying weights, meaning measurements of mass (such as grams) can be misleading when compared the reactions of individual molecules. On the other hand, using the absolute number of atoms/molecules/ions would also be confusing, as the massive numbers involved would make it all too easy to misplace a value or drop a digit. As such, working in moles allows scientists to refer to a specific quantity of molecules or atoms without resorting to excessively large numbers.