Why is Ionization Energy definition restricted to gaseous atoms? Here's the definition of ionization energy that I am learning at this level: 


  energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of uni positively charged gaseous ions.


I would like to know, why did they specify that the removal of electrons have to be at gaseous state and the ions that form are gaseous as well? Does it have to do with the amount of energy supplied that causes the elements to vapourize?
In fact, it is not restricted at all. You may compare the energies needed to remove one electron out of a solid; these are also meaningful, albeit in a different way, and known for a wide range of substances. But when you are talking about atoms, you want to measure atoms, and the only way to have an undisturbed lone atom is to put it in a gaseous state. And of course, the resulting ion should be in that state, too; after all, you can't make a solid out of positive ions alone, and if you want to add negative ions to counterbalance them, an ambiguity arises as to which ions should you use.