Zeus is the one Greek god whose name can be traced with confidence to the ancestral language behind Greek, Latin, Sanskrit and much of Europe. Zeus derives from a Proto-Indo-European sky-father, Dyeus phter, the same root that gives Latin Iuppiter (Jupiter), Vedic Dyaus Pita and, through a different path, the ordinary word for day. He is, at his oldest layer, the bright daytime sky personified, and his standing weapon, the thunderbolt, belongs to that origin.
In the developed religion he is king of the gods rather than their maker. He did not create the cosmos; he seized and then stabilised its rulership. His authority is real but not unlimited: he is bound by Moira (fate or allotment), and Homer shows him unable simply to overrule what is fated even for a son he loves.
His epithets map his functions precisely. Zeus Xenios guards the guest and the stranger; Zeus Horkios witnesses oaths; Zeus Herkeios protects the household boundary; Zeus Ktesios guards the storeroom. To the Greeks the same god who threw lightning across the Aegean also stood, small and domestic, beside the family hearth. This double scale, cosmic sovereign and household protector, is characteristic of how the Olympians were actually worshipped.