The pater familias (plural: patres familias) was the highest ranking family status (status familiae) in an Ancient Roman household, always a male position. The term is Latin, literally, for "father of the family". The form is irregular and archaic in Latin, preserving the old genitive ending in -as (see Latin declension). In contemporary English, the term is sometimes used to refer to a person who is acknowledged as exercising the most authority within a family, household, or group.
The Roman pater was not a father in the modern, mostly western, sense of the concept, but a chief of the family domus (house). Pater is thus a distinct concept from that of the biological father, which was called the Genitor. The power held by the pater familias was called patria potestas (paternal power). Potestas is distinct from auctoritas, also held by the pater. The power of the pater was over his familia iure proprio (not necessarily kin-based, but a political, economical and religious unit) and his familia domestica (based on kinship and co-residence).
He had the power to sell his children into slavery; Roman law provided, however, that if a child has been sold as a slave three times, he is no longer subject to the patria potestas. The pater familias had the power to approve or reject marriages of his sons and daughters; however, an edict of the Emperor Caesar Augustus provided that the pater familias could not withhold that permission lightly.
One should notice that the pater's children, the filii familias, could be other than biological offspring, such as brothers, nephews or adoptive sons and daughters. In Ancient Rome, the family household was, therefore, conceived as an economical and juridical unit subordinated to a single person, with a great deal of authority (the potestas and auctoritas) over all its members - in fact, the Latin word familia (which is the etymological origin for the English word "family"), originally meant the group of the famuli (servus or serfs and slaves) living under the same roof. And the familia was considered the basic social unit, more primordial, for instance, than the gens (clan, caste, or group of families).
Besides being a chief, the pater familias was the only person endowed with legal capacity, or sui iuris. Women (in most but not all cases), the filii, slaves and foreigners had a capitis deminutio (literally, a "diminished head", meaning diminished capacity), that is, they could not celebrate valid contracts, nor did they possess, by rule, personal property. All assets and contracts belonged, in principle, to the pater. A capitis deminutio meant a tendential lack of legal personality, even if there were some restrictions: there were laws protecting the slaves, and the incapable (everyone with a capitis deminutio) could, in some circumstances, possess a quasi- personal property, the peculium.
As such, the patres familias were the only full legal persons, but, because of their extended rights (their longa manus, literally "long hand"), they also had a series of extra duties: duties towards the women, the filii and the slaves (though some of these duties were not recognized by the original ius civile, but only by the ius gentium, specially directed to foreigners, or by the ius honorarium, the law of the Magistratus, specially the Praetor, which emerges in a latter period of Roman law).
Only a Roman citizen, someone with status civitatis, could enjoy the status of pater familias. There could only be one holder of the office within a household. Even male adult filii remained under the authority of their pater while he still lived, and could not acquire the rights of a pater familias while he was yet alive; at least in legal theory, all their property was acquired on behalf of their father, and he, not they, had ultimate authority to dispose of it. Those who lived in their own households at the time of the pater's death succeeded to the status of pater familias over their respective households (pater familias sui iuris), even if they were just in their teens. Women were always under the control (sub manu) of a pater familias, either their original pater, or the pater of their husband's family once married (which could be her husband or not).
Over time, the absolute authority of the pater familias tended to be weakened, and rights that theoretically existed were no longer enforced or insisted upon. The power over life and death was abolished, the right of punishment was moderated, and the sale of children was restricted to cases of extreme necessity.