Originally, a patriarch was a man who exercised autocratic authority as a pater familias over an extended family. The system of such rule of families by senior males is termed patriarchy.

This is a Greek word, a compound of πατριά (patria), “lineage, progeny”, esp. by the father’s side (which derives from the word πατήρ – patēr meaning “father”) and ἄρχων meaning “leader”, “chief”, “ruler”, “king”, etc.