Final answer:
External obsolescence refers to the decrease in the value or usefulness of a property due to factors beyond the control of the homeowner. It is considered incurable as it cannot be fixed by the homeowner.
Step-by-step explanation:
External obsolescence, by definition, refers to the decrease in the value or usefulness of a property due to factors beyond the control of the homeowner, such as changes in the surrounding area or market conditions. It is considered incurable because it cannot be remedied by the homeowner. Examples of external obsolescence can include noise pollution, proximity to undesirable facilities, or changes in zoning regulations.