Thermal boundary layer
Here are the facts : the fluid particles' temperature in direct
contact with the solid is equal to the solid surface's temperature,
whereas this is not the case in the middle of the flow (where generally
temperature is minimal when the fluid cools the solid). Temperature
won't go suddenly from its value at the surface to its minimum,
but it will be achieved gradually, and this occurs in this boundary
layer. Here is where the effects of the fluid's thermal conductivity
take place.
The following picture shows a laminar thermal boundary layer, it
was obtained by differential interferometry (the heated fluid has
a density and an refraction index which varies).

Laminar thermal
boundary layer, photo from the book " An album of fluid motion,
Van Dyke, The Parabolic Press "
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