Boundary Layer
Definition
The thin skin of slow-moving fluid hugging every surface — the reason golf balls have dimples.
Technical definition
The viscous region near a wall where velocity rises from zero (no-slip condition) to the freestream value; thickness grows roughly as δ ~ √(νx/U) and controls drag.
Example
Golf ball dimples trip the boundary layer turbulent, delaying separation and cutting drag — adding roughly 50% to drive distance.
Common confusion
The boundary layer isn't where 'friction happens instead of flow' — the whole flow obeys the same equations; the layer is just where viscosity's effect concentrates.