The soul was often described as the spiritual essence of an individual. Many cultures had a concept of a soul, but there were different interpretations. Many religions on Earth had a concept of a soul.
In the mirror universe, Humans didn't believe in the soul. (DS9: "Crossover") The Ocampan race believed in a soul to some extent; they referred to it as the comra. (VOY: "Emanations") The Vulcan soul was known as a Katra.