Wiktionary
arrested decay
n. State of preservation where no attempt is made to improve a ruin, but rather to keep it from deteriorate any further.
Wikipedia
Arrested decay
"Arrested decay" is a term coined by the State of California, United States to explain how it would preserve its Bodie State Historic Park. A more common application of this concept is the preservation of war ruins as memorials.
Usage examples of "arrested decay".
They are lean, furtive streets, gray as the January sky with a sort of arrested decay.
He could almost smell the medical school's dead house with its stink of formalin, its corpses in a state of semi-arrested decay.