A habit was something a person did on the regular, and the person found it hard to give up. However, unlike an addiction, the person did not suffer harmful physical effects if they refrained from doing it.
The Akaali apothecary Riaan had the old habit to mutter when writing while Jonathan Archer had the habit to talk to his dog. They both told each other while observating Garos' curio shop at night. (ENT: "Civilization")
According to Captain Jonathan Archer, Humans had the peculiar habit of talking during their meals. Trip Tucker later told T'Pol that she maybe picked up some of the Human habits when she told him that her subconscious mind controls her decisions. (ENT: "Breaking the Ice")
After Seven of Nine returned from being kidnapped by the Borg, she was ordered to regenerate for two days, but disobeyed. Kathryn Janeway told her that she had picked up some bad habits. (VOY: "Dark Frontier")