What is the fallacy begging the question?

'Begging the question' (petitio principii) is the fallacy of making the answer to a question part of the question itself.

In the days before women began to speak for themselves many men argued that:

"No real woman would even want to go out to work. She would rather stay at home and keep house for her husband and family."

If you pointed out that Florence Nightingale, Marie Curie and even Joan of Arc all found better things to do than 'stay at home and keep house for their husband', the person making the case could say "Yes, but those people were not real women.".

Perhaps Florence Nightingale, Marie Curie and Joan of Arc were really men in women's bodies, perhaps they were Androids, or possibly hippopotamuses - someone who 'begs the question' already knows what the truth is:- they are hardly going to be hindered by reality.