Chronic
This important term in medicine comes from the Greek chronos, time and means lasting a long time.
A chronic condition is one lasting 3 months or more, by the definition of the U.S.
National Center for Health Statistics.
In ancient Greece, the 'father of medicine' Hippocrates distinguished diseases that were acute (abrupt, sharp and brief) from those that were chronic.
This is still a very useful distinction.
Subacute has been coined to designate the mid-ground between acute and chronic.