means To rule as a monarch, or the period during which such a ruler holds power; more loosely, to be the dominant force in some realm.

from From Latin 'regnum' (kingship, rule), built on 'rex,' king — a relative of 'regere,' to guide or keep straight, the same root that steers 'regent,' 'regal,' and 'regulate.' It reached English through Old French 'reigne.' That stray 'g' is a Latin-loving spelling flourish; the word has nothing to do with 'rein' (the horse-strap, from Latin 'retinere,' to hold back) or 'rain' — though the homophones quietly agree that a ruler both restrains and pours down.