The vertical downward movement of a structure's base is known as _____. 

Explanation:

1. Settlement refers to the vertical downward movement of a structure's base, which occurs due to the compression of the soil beneath the foundation or due to the weight of the structure itself.

2. This is a natural response to loading and can vary depending on the soil type, moisture content, and other environmental factors.

Additional Information

1. Deformation: A general term for the change in shape or size of a material or structure due to applied forces, but not specifically referring to the downward movement of a foundation.

2. Deflection: Refers to the displacement or bending of structural elements under load, typically in beams or slabs, rather than the movement of the base.

3. Depression: Refers to a general lowering or sinking, but it is not commonly used in structural engineering to describe the movement of the foundation.