Holo-addiction was a psychological disorder in which a person preferred the fictional world of the holodeck to the real world. The addiction functioned as 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 real friends. (TNG: "Hollow Pursuits") 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. (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")