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Boundary layer is defined as

Boundary layer is defined as Correct Answer A thin layer at the surface where gradients of both velocity and temperature are large

Explanation:

A boundary layer is a thin layer of viscous fluid close to the solid surface of a wall in contact with a moving stream.

In scientific terms, A boundary layer is a thin region in the fluid adjacent to a surface where velocity and temperature gradients normal to the surface are significant.

The difference in velocity between adjacent layers of the fluid is known as a velocity gradient.

The difference in temperature between adjacent layers of the fluid is known as a temperature gradient.

The boundary layer thickness (δ) is the distance across a boundary layer from the wall to a point where the flow velocity has essentially reached the free stream velocity.

Thus velocity gradient will be large and at the surface, due to less velocity difference, it will be a thin shear layer.

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Similarly, the thermal boundary layer exists in heat transfer.

Multiple types of boundary layers can coexist near a surface simultaneously. 

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