The soul, or spirit, 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 2151, Trip Tucker asked T'Pol if she wanted a bite of his pecan pie to taste. She refused as the dessert was "mostly sugar." Trip retorted that "Vulcans don't have a sweet tooth" and that it "may not be good for the body, but it sure is good for the soul." (ENT: "Breaking the Ice")
When Captain Gabriel Lorca confessed to having killed the crew of his previous command, the USS Buran, to prevent them from having to endure Klingon torture, Harcourt Fenton Mudd commented that confession was good for the soul, but it was too bad none of them had one anymore. (DIS: "Choose Your Pain")
In ruling on the question of whether Commander Data was the property of Starfleet, Captain Phillipa Louvois stated that she did not know whether Data had a soul or even if she had, but ruled that he had the freedom to explore the question himself. (TNG: "The Measure Of A Man")
According to Data, his fellow violinists felt that he was technically proficient, but lacked soul. (TNG: "The Ensigns of Command")
In offering his services as a guide to the crew of the USS Enterprise-D, Q warned Captain Jean-Luc Picard that the adversaries they had faced were nothing compared to what was coming and that they were about to enter areas "containing wonders more incredible than you can possibly imagine, and terrors to freeze your soul." When Picard rejected him, however, he catapulted the Enterprise into an encounter with the Borg, one of the terrors he had tried to warn them about. (TNG: "Q Who")
Roga Danar stated that even with the overwhelming demand which had been built into his soul to survive by the conditioning he had undergone to participate in the Tarsian War, he would rather die than return to the penal colony on Lunar V. (TNG: "The Hunted")
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." Still 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")
Major Kira Nerys worried that Kai Opaka thought her to be a violent person without a soul, without a conscience. Opaka, however, told her not to deny the violence inside herself and that the Prophets were only waiting for her to forgive herself. (DS9: "Battle Lines")
In the mirror universe, Benjamin Sisko told the prime universe's Kira Nerys that Terrans didn't believe in the soul. (DS9: "Crossover")
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 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")
The members of Chakotay's tribe believed in the soul. The medicine wheel could be used to guide the soul based on the placement of the Coyote Stone. Placing this stone between the crossroads of the Fifth and Sixth Realms was believed to divert the soul into the Mountains of the Antelope Women. (VOY: "Cathexis")
The Ocampan race believed in a soul to some extent; they referred to it as the comra. (VOY: "Emanations")
The Vulcan Katra was described as "what some people would call a soul", and was often compared to it. (Star Trek IV: The Voyage Home; VOY: "Innocence"; DIS: "Lethe")
For not accepting his theory of cellular ennui, Dr. Elias Giger derided the general scientific establishment as "soulless minions of orthodoxy." (DS9: "In the Cards")
There was a saying that the eye is the window to the soul. (VOY: "The Gift")