(covers information from several alternate timelines)
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 describing the panic accompanying the prophesied return of Ardra on Ventax II, Doctor Howard Clark explained that the people there believed that they had sold their souls to the devil. Later, when an impostor posing as Ardra proposed terms for arbitration regarding her claim to the planet with Captain Jean-Luc Picard, she demanded a steep price of him if he lost; she wanted "your heart, your mind, your soul, and I want you to give them to me without resistance, of your own free will." Later, in describing his dilemma to Lieutenant commander Geordi La Forge, Picard told him "my reputation as a litigator, not to mention my immortal soul, is in serious jeopardy." (TNG: "Devil's Due")
Bareil Antos felt that "When you overindulge the body, you starve the soul." When Antos's mirror universe counterpart visited Deep Space 9, Major Kira Nerys took a great interest in him, having been the lover of the Bareil Antos from the prime universe. Unfortunately, the mirror Antos had been sent there to steal an Orb for the the Intendant. When Kira tried to stop him from doing this, the Intendant (Nerys's own mirror universe counterpart) commented, "Isn't that sweet? She is still trying to save your soul. Of course, you don't have one." (DS9: "Resurrection")