Holo-addiction is a psychological disorder in which a person spends time in the fictional world of the holodeck in preference to the real world. This phenomenon is usually an escape mechanism symptomatic of deeper psychological problems.
Lieutenant Reginald Barclay suffered from holo-addiction when he was assigned to the USS Enterprise-D in 2366, which was cured when he started to make friends. He had a relapse in 2376, after he had left the Enterprise, when he re-created the USS Voyager and its crew. After he succeeded in contacting the real Voyager, he recovered, and began dating a woman named Hope. (TNG: "Hollow Pursuits"; VOY: "Pathfinder")
While conducting research on Human interactions in 2378, Seven of Nine began to experience a form of holo-addiction, causing her to be absent from her post on several key occasions. When confronted by Captain Janeway about it, Janeway noted that she logged 49 hours in Holodeck 2 in a six-day period of time. In response to the confrontation, Seven falsely stated that she was "running a simulation of a new gravimetric array" she had been developing. (VOY: "Human Error")