This is known as the fallacy of petitio principii (begging the question / circular logic). (religion.blogs.cnn.com)
There are two meanings of this expression - the original version and how the expression has come to be used by many: «Begging the question» derives from the Latin petitio principii - «a request for the beginning or premise». (bookbrowse.com)
«It's a translation of the Latin phrase petitio principii, and it's used to mean that someone has made a conclusion based on a premise that lacks support,» writes Grammar Girl, who explains the complicated subject well in her blog. (inc.com)