Mean velocity gradient is the rate of change of velocity per unit distance normal to a section. It is an important concept in fluid mechanics and is used to describe the velocity profile and flow characteristics of a fluid.

Explanation:
1. Mean Velocity:
- Mean velocity refers to the average velocity of a fluid over a given distance or section.
- It is calculated by dividing the total distance traveled by the fluid by the total time taken.
- Mean velocity is a scalar quantity and does not provide information about the velocity distribution within the fluid.

2. Average Velocity:
- Average velocity is similar to mean velocity and refers to the average rate of change of displacement over a given time interval.
- It is calculated by dividing the total displacement by the total time taken.
- Average velocity is also a scalar quantity and does not provide information about the velocity distribution within the fluid.

3. Velocity Gradient:
- Velocity gradient, also known as the shear rate, is a measure of the change in velocity per unit distance normal to a section.
- It describes how the velocity of the fluid changes as we move away from a specific point or section.
- Velocity gradient is a vector quantity and provides information about the velocity distribution within the fluid.

4. Mean Velocity Gradient:
- Mean velocity gradient is the average rate of change of velocity per unit distance normal to a section.
- It is calculated by dividing the change in velocity by the distance normal to the section.
- Mean velocity gradient is a measure of the velocity variation within the fluid and is commonly used in the analysis of fluid flow.

Conclusion:
The rate of change of velocity per unit distance normal to a section is called the mean velocity gradient. It provides information about the velocity distribution within the fluid and is an important parameter in fluid mechanics.