Originally, Christendom was used as a term meaning 'the Christian World', so basically all the people who were Christian. It also referred to Christianity itself, such as in Norwegian where the word Kristendom means Christianity. As Christianity began to completely dominate the world, however, the meaning of it changed. By the mid-late Dark Ages, Christendom was used by the middle European powers to describe what they saw as the centre of the civilised world. Of course the "civilised world" could only include Christians. The following is a list of countries within Christendom during its various stages:

1This is not to suggest that they were solid states with defined borders as we know today, but rather masses of hotly contested lands where small warlords and dukes reigned.

After this the word Christendom, when referring to a specific area, wasn't really used. The colonies (i.e. America, Australia etc.) were never included in this term, and Christendom wasn't really related to imperialism. Eventually the word returned to its original meaning, "the Christian World", all those who are Christian. Since this time Christendom has been in decline, as the harsh enforcement of it has ended, and there is much more toleration of alternative religions.