Definition of Fictio
In Roman law, a fiction; an assumption or supposition of the law. Such was properly a term of pleading, and signified a false averment on the part of the plaintiff which the defendant was not allowed to traverse; as that the plaintiff was a Roman citizen, when in truth he was a foreigner. The object of the Fiction was to give the court jurisdiction.
That's the definition of Fictio in Black's Law Dictionary 6th Edition. Courtesy of Cekhukum.com.