Begging the Question
That is, that the proposition to be derived is used to derive itself.
Then the fallacy of begging the question has been perpetrated.
The term begging the question originated as a 16th century mistranslation of Latin petitio principii, which means assuming the initial point.
Consequently there is considerable confusion in natural language as to exactly what this phrase actually means.
In contemporary and colloquial language it is commonly used to mean raising the question, in particular to call attention to a question or a point for debate which has conspicuously not been raised but, in the questioner's mind, ought to have been, as has been made apparent during the process of the argument under discussion. The cliché "elephant in the room" can frequently be seen in close proximity, and should arguably be used instead.