'''Christendom''', according to the Merriam-Webster dictionary, was the name for the medieval Christian empire around the 12th century.<ref>http://www.merriam-webster.com/dictionary/Christendom</ref>
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'''Christendom''', is the geographical part of the world in which Christianity prevails or which is governed principally under Christian institutions; it can also mean Christianity as an organized religion. "Thomas Aquinas's books were read throughout Christendom."