monarch

Monarch comes from Middle French monarque, from Late Latin monarcha, from Ancient Greek monárkhēs (a variant of mónarkhos, 'sole ruler'), itself built from mónos, 'only,' plus arkhós, 'leader.'

The facts point to a single, stable meaning across this chain: from Greek onward, the word has always named someone who rules alone.

It shares its Greek roots with other English words built from mono- ('only') and -arch ('leader' or 'ruler'), the same pieces that formed monárkhēs.