genius, in Roman religion, guardian spirit of a man, a family, or a state. In some instances, a place, a city, or an institution had its genius. As the guardian spirit of an individual, the genius (corresponding to the Greek demon) was largely the force of one's natural desires. The genius of the paterfamilias was honored in familial worship as a household god and was thought to perpetuate a family through many generations. Notable achievements or high intellectual powers of an individual were attributed to his genius, and ultimately a man of achievements was said to have genius or to be a genius.

In ancient Roman religion, the attendant spirit of a person or place. It originally represented the housefather, who gave continuity to a family or clan over generations; its female counterpart was the housemother, called the juno. In later times, the genius was seen as an individual's guardian spirit or higher self, and it was worshiped by that individual, especially on his birthday. There were also genii of places and of groups such as legions, states, and guilds.

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