Laminar flow is non-turbulent flow in smooth parallel (non-intersecting) paths in layers that have different velocities. There are no cross-currents perpendicular to the direction of flow, nor eddies or swirls of fluids.
The term laminar flow describes one of the three types (the other two are transitional and turbulent) of behavior that a boundary layer can exhibit.
When a fluid (such as air) moves (or flows) past a solid surface, such as an airplane wing, a thin layer develops adjacent to the surface where frictional forces tend to retard the motion of the fluid. This layer is defined as the boundary layer.
In general, the most desirable state is one with a high degree of laminar flow, i.e., exhibits a low degree of friction drag. In particular, a high degree of laminar flow reduces the amount of fuel consumed by an aircraft and also increases its flight range.
The process of a laminar fluid flow becoming turbulent is known as a laminar-turbulent transition (or transitional flow). The main parameter characterizing transition is the Reynolds number.
Each of these flows behaves in different manners in terms of their frictional energy loss while flowing and have different equations that predict their behavior. Transition is often described as a process proceeding through a series of stages. “Transitional flow” can refer to transition in either direction, that is laminar-turbulent transitional or turbulent-laminar, transitional flow.