Loneliness was the unhappiness or depression an individual felt when they had no friends or did not have someone to talk to.
In 2266, in her conversation with Captain James T. Kirk aboard the USS Enterprise, Eve McHuron rationalized the behavior of the male crewmembers, who were looking at her and following her with their eyes, as them being lonely. Then, Eve said she could understand loneliness and compared her state with that of Kirk. She thought he would understand loneliness more, saying, "... having to run a huge ship like this with so much responsibility every minute and having to be so careful with all your men looking up to you." (TOS: "Mudd's Women")