Another definition from the internet:

"The Latin term for the fallacy is petitio principii, a
translation of the Greek to en archei aiteisthai="at the
beginning to assume"; but aiteisthai does literally mean "to beg".
The phrase can be traced back to Aristotle (4th century B.C.):
"Begging or assuming the point at issue consists (to take the
expression in its widest sense) in failing to demonstrate the
required proposition. But there are several other ways in which
this may happen; for example, if the argument has not taken
syllogistic form at all [...]. If, however, the relation of B to C
is such that they are identical, or that they are clearly
convertible, or that one applies to the other, then he is begging
the point at issue." (Prior Analytics II xvi)
"