Rather recent onset or somewhat rapid change.
The term "subacute" is used in contrast to acute which indicates very sudden onset or rapid change and chronic which indicates indefinite duration or virtually no change.
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.