A repressed memory was a term used to describe the brain suppressing a traumatic event in one's life. To draw the memory out instead of suppressing it, a Vulcan normally needed a pyllora, a term used for a guide, to help draw it out.
In 2373, Lieutenant Tuvok had flashes of a supposed memory he had of trying to help a young girl from falling from a precipice. Tuvok was not able to pull her up and she fell to her death. Tuvok was unable to recall the incident, so he enlisted the help of his friend, Captain Kathryn Janeway, to act as his pyllora. The Doctor later discovered that a memory virus was infecting Tuvok's brain, which had been transmitted to him from his former crewmate from the USS Excelsior, Lieutenant Commander Dmitri Valtane, in 2293. The virus masqueraded as a traumatic event, so the brain would try to suppress and hide it. The Doctor was later able to destroy the virus with thoron radiation. (VOY: "Flashback")