Depression, or sometimes clinicla depression was a mood disorder that caused a persistent feeling of sadness and loss of interest. It often followed a traumatic event, such as a death of someone close. Depression could also be caused by biochemical imbalances.
Aboard the USS Voyager, B'Elanna Torres believed she had lost all the family she ever had after Chakotay informed her of the Maquis massacre. Torres discovered that she felt no overwhelming pain over the death of some of her closest friends, and began running dangerous holodeck programs without the safety protocols in order to inflict physical pain on herself to satisfy her feelings of guilt. After Chakotay confronted Torres in the holodeck with a recreation of the massacre, she eventually dealt with the feelings by helping the Voyager crew on a daring mission to rescue a multispatial probe aboard the newly-built Delta Flyer. (VOY: "Extreme Risk")
In 2371, upon the sudden accidental deaths of his brother Robert and his nephew René, the issue of lineage and lack of offspring caused a sustained yet brief period of depression for Captain Jean-Luc Picard. (Star Trek Generations)
Following the death of her father, Kathryn Janeway spent months in bed sleeping rather than dealing with her emotions. Her sister was ultimately responsible for helping her move on with life. (VOY: "Coda")