A boundary layer is the thin layer of fluid right next to a surface where friction and viscosity significantly affect flow behavior.

Importance in science and engineering:

• Determines drag on aircraft, cars, and ships
• Influences heat transfer in engines and cooling systems
• Affects weather patterns and wind behavior over land and oceans

Example:
Airflow slows down when it touches an airplane wing due to friction, forming a boundary layer that engineers must manage for efficient flight.