(informal) Ingenious, brilliant, very clever, or original.

Someone possessing extraordinary intelligence or skill; especially somebody who has demonstrated this by a creative or original work in science, music, art etc. / Extraordinary mental capacity.

The tutelary deity or spirit watching over a particular person.

A person who is very intelligent, but socially dull. / A person who is very intelligent in one limited sphere, but inept in other areas.

The spirit each person is believed to have in attendance, according to certain religious or mythological traditions, which tries to negatively influence him, and is opposed by his good genius; loosely, someone who is a bad influence.

(rare) A single spirit or minor deity (genius) which watches over several places, rather than only one.

(Roman mythology) The minor deity or spirit watching over a particular place. / (figuratively) The spirit or atmosphere of a place, especially conceived as a source of artistic inspiration.

A sudden mental insight that solves a problem.

A spirit presiding over or guarding a family or household.

(obsolete) An evil spirit or angel, seen as influencing a person's decisions etc.