What term describes something that has decayed or fallen into disrepair?

The term that describes something that has decayed or fallen into disrepair is "dilapidate." When a structure or object is in a state of dilapidation, it indicates that it has deteriorated due to neglect, age, or lack of maintenance. This word specifically captures the essence of decay and a condition where the original form has significantly diminished, often making the object unsafe or unfit for use.

In contrast, rejuvenate refers to restoring something to a more youthful or fresh state, while maintain implies keeping something in its current state through regular upkeep. Fortify suggests strengthening or enhancing the security or stability of something, which is unrelated to degradation or decay. Thus, "dilapidate" is the most appropriate term for describing something that has deteriorated or deteriorated.