This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
A therapist was a qualified psychologist who treated the psychological issues of individuals. Some, like Beverly Crusher, believed that therapists were the worst patients, second only to doctors. (TNG: "The Loss") The private life of therapists was considered as off-limits to patients. For example, it was considered inappropriate for a patient to follow their therapist on vacation. (VOY: "Inside Man") There was an expectation by some that therapists were individuals who were advanced in age. (DS9: "Afterimage")
The Tantalus Penal Colony employed several therapists, including a man named Eli. In the mid-23rd century, after her rehabilitation there, Lethe became a therapist for her fellow inmates. Dr. Adams praised her skill to James T. Kirk and Helen Noel. Later, another therapist explained the workings of the neural neutralizer to Kirk. (TOS: "Dagger of the Mind")
In 2370, when Data was visiting Deanna Troi aboard the USS Enterprise-D, Juliana Tainer mistakenly believed that Data was having a sexual relationship with Troi. When Data recognized Juliana's mistake, he informed her that Troi was the ship's therapist. (TNG: "Inheritance")
In 2374, a Federation magistrate wanted Molly O'Brien evaluated for her issues at a special care center on Dalvos Prime. There was an expectation that Molly would be staying long-term at the facility. Her father, Miles O'Brien, expressed his dissatisfaction with the magistrate's suggestion. Miles desired that his daughter form a connection with him and her mother, not some therapist Molly did not know. (DS9: "Time's Orphan")
That same year, The Doctor integrated the psychological approach of two methods for Seven of Nine's memory reconstruction. One of these methods was the approach used by Jungian therapists, who attempted to retrieve unconscious memories by exploring synchronicities in recent events. (VOY: "Retrospect")
Both Jake Sisko and Quark expressed surprise in 2375 that Ezri Dax was a therapist. When she asked the Ferengi why everybody was so surprised to hear it, he attirbuted it to her youth. (DS9: "Shadows and Symbols", "Afterimage")
In 2376, Deanna Troi expressed her frustration with The Doctor and Lewis Zimmerman's constant personality clashes (especially given that The Doctor was trying to save Zimmerman, his creator and template's, life ) saying she'd need " a whole team of therapists just to get them in the same room.". (VOY: "Life Line")