What is Atavus?

Legal Definition
The male ascendant in the fifth degree, was so called among the Romans, and in tables of genealogy the term is still employed.
-- Bouviers Law Dictionary
Legal Definition
The great-grandfather's or great-grandmother's grandfather; a fourth grandfather. The ascending line of lineal ancestry runs thus: Pater, Avus, Proavus, Abavus, Atavus, Tritavus. The seventh generation in the ascending scale will be Tritavl-pater, and the next above it Proavi-atavus.
-- Black's Law Dictionary
Legal Definition
The male ascendant in the fifth degree.
-- Ballentine's Law Dictionary