The genius is the vital spirit that accompanied a man from his birth to his death. The genius of a person was honored on his birthday. By the beginning of the Empire, a woman's genius, called the juno was also honoured.
Genius populi Romani
There was "a shrine dedicated to the Genius of the Roman people, near the temple of Concord in the forum" .
The genius loci was the genius associated with a specific place. A graffito found in a excubitorium in Ostia "mentions the Genius of the place..." 
See also: "Genius" at Wikipedia