A chronic condition was a medical condition that recurred often or lasted a long time.
In 2369, Krax described his father Zek's (supposed) cause of death as an infection of the tympanic membrane, a chronic condition. (DS9: "The Nagus")
In 2375, after Kathryn Janeway remarked on a recovering B'Elanna Torres's Klingon temper having returned, The Doctor replied that "it practically qualifies as a chronic condition". (VOY: "Nothing Human")
"Chronic illness" was mentioned on a form which may or may not have been seen in the Star Trek: Enterprise third season episode "Carpenter Street".This form can be seen at YourProps.com.