What is Exact mass?

The monoisotopic mass of an ion is the sum of the exact masses of its most abundant isotopes ($\ce{^{12}C}$ = 12 exactly, $\ce{^1H}$ = 1.007825, $\ce{^{16}O}$ = 15.994915, $\ce{^{14}N}$ = 14.003074 u). High-resolution mass spectrometry measures $m/z$ to a few thousandths of a unit or better, enough to tell ions of the same nominal mass but different formulas apart.