Subsidence is downward movement of the ground beneath a building that is not caused by the weight of the building itself — for example soil shrinkage, washing out of fines by a leaking drain, or collapse of an underground void. It differs from normal settlement, which is the ground compressing under new load.
The distinction matters because the causes and remedies differ. Settlement occurs soon after construction as the ground compresses under the new load and usually stabilises. Subsidence occurs later and is driven by something changing in the ground: moisture loss in clay, erosion by leaking water, dissolution of limestone forming karst features, or old mine workings and voids.
Investigation normally starts by identifying and removing the cause — repairing drains, managing trees, or grouting voids — before any structural repair. Underpinning is sometimes necessary but should follow diagnosis, not precede it.