what i have been taught is exactly what mrdeadman said, that the ionization energy was the energy required to remove an electron/steal an electron thus making it an ion which is that same as the wikipedia definition
here's the address if you want to read it http://en.wikipedia.org/wiki/Ionization_energy
That is a perfectly acceptable general idea of what the ionization energy means, but as a definition it is a precise measurement that has to have a standard, and the one used is normally the one I mentioned. Otherwise, the ionization energy would be impractically low to work with! For example, the first ionization energy of an atom of Barium would be 0.00000000000000000083508 Joules!
Thats why they use moles.