Hygroscopic water is the correct term used to describe the water that remains in the soil after reaching the permanent wilting point. Here is a detailed explanation of the answer:

Permanent Wilting Point:
The permanent wilting point (PWP) is the soil moisture content at which plants can no longer extract water from the soil, and they wilt permanently. At this point, the soil is considered to be at its driest state and the remaining water is tightly held by the soil particles.

Explanation:
Hygroscopic water refers to the water that is held in the soil by the forces of adsorption. It is the water that remains bound to the soil particles even when the soil is at its driest state. This water is strongly attracted to the soil particles and is not available for plant uptake.

Key Points:
- The other options mentioned in the question are not correct because they refer to different types of soil water.
- Delta refers to the change in a variable, and it is not related to soil water.
- Capillary water refers to the water that is held in the soil against the force of gravity, and it is available for plant uptake.
- Gravity water refers to the water that drains through the soil under the force of gravity, and it is not available for plant uptake.

Conclusion:
In summary, the water that remains in the soil after reaching the permanent wilting point is called hygroscopic water. This water is tightly held by the soil particles and is not available for plant uptake. Understanding the different types of soil water is important in agriculture and civil engineering to optimize irrigation practices and manage water resources effectively.