pater

Pater, patris is third declension masculine and shortens its stem exactly as frater and mater do: patris, patrem, patre, patres, patrum, patribus.

Its plural carries a second meaning that dominates Roman political prose. Patres are the senators, and patres conscripti, "conscript fathers", is the formula an orator uses to address the senate; almost every speech of Cicero's opens with it. Read patres in a political passage as "the senate" first and "the fathers" second.

Pater familias preserves an archaic genitive: familias, not familiae and certainly not an accusative plural. The phrase is a fossil, and knowing that it is one saves you trying to parse it. Off the same stem come patria, "the fatherland", patrimonium, "inheritance", and patronus. English took paternal, patriot, patrician, patron and expatriate, and Spanish padre, Italian padre and French pere are the word itself, worn down.