Morale officer was a term for the person whose job it was to look after the morale of a crew.
Neelix, a non-Starfleet member of the USS Voyager crew, was the ship's morale officer. He had duties that coincided with those typically associated with the ship's counselor. This title was self proclaimed by Neelix in 2371. (VOY: "The Cloud"). However, it had been accepted by Captain Kathryn Janeway by 2372. (VOY: "Initiations") As part of his duties, he made a point of researching the various holidays Voyager's crew celebrated, including Vulcan holidays such as Kal Rekk. (VOY: "Meld")
After Neelix's success with the Tak Tak in 2373, Janeway commented that she might have to promote him from morale officer to ambassador. (VOY: "Macrocosm") Neelix retained the position of morale officer until his departure in 2378. (VOY: "Homestead")
In 2374, Ensign Nog told Lieutenant Commander Worf that he was glad he was not the morale officer of Deep Space 9 or the USS Defiant, as the Klingon had told him that the increasing numbers of Jem'Hadar would only be a concern for him if he survived the attempt to recapture the Defiant. (DS9: "One Little Ship")
In 2375, the Silver Blood duplicate of Neelix aboard the duplicate Voyager was promoted to chief medical officer by Janeway following the loss of The Doctor's program. When he asked her about his "other duties," she told him to focus on Sickbay, as morale was a luxury at that point. (VOY: "Course: Oblivion")