Pronounced day fact-o, this widely used expression means "in fact" or "in reality". So why use it at all?
As is often the case in modern use of Latin expressions, context is everything. It may "in fact" be true, but often it is not officially recognised as being so.
e.g. "Miguel Indurain was the de facto team leader after Pedro Delgado's disastrous time trial."
In the 1989 Tour de France, the professional Banesto cycling team's leader was Pedro Delgado, who had won the race in 1988. However, Miguel Indurain was riding better and was the best placed member of the team, even though Delgado was still officially team leader.