A letter was a handwritten or typed document addressed to a specific individual or group.
In 2267, Jame Finney apologized to James T. Kirk for her outburst, saying she realized how close he and her father had been upon reading about him in letters her father had written to her mother and her. (TOS: "Court Martial")
In 2374, the crew of USS Voyager received letters from their families and friends via the Hirogen communications network. Tuvok from his wife T'Pel and Kathryn Janeway from her former fiancé, Mark Johnson. (VOY: "Hunters")