Laminar flow is important stuff in more fields and more fluids than one would at first guess. Laminar flow of any fluid agrees roughly with our instinctive sense of "smooth," non-turbulent flow; flow free of swirls, eddies, back currents, or other disturbances. David Hedrick's image of boards on rollers is quite helpful for getting the idea of what physicists call a "velocity gradient." We can extend the idea to pipes, shifting our point of view so that we're looking straight down the pipe (seeing it in cross-section). Then the stationary layer is the layer (the cylindrical layer) against the wall, with a layer inside moving a little faster, the next a little faster, etc., until the fastest flow is in the center. VERY IMPORTANT TO REALIZE: (1) there are really no "layers": the velocity gradient is continuous from wall to center; and (2) in a theoretically perfect laminar flow, there is no friction with the wall -- there is a micro "layer" that doesn't move at all. And then we can shift point of view again, looking instead at an airplane wing end-on (as though we were standing beside the airplane's wing, looking along it at the fuselage). Now the critical fluid flow (air being a fluid also) is from the front of the wing, where the oncoming in-flight airstream divides to flow over and under the wing, to the rear, where that stream recombines. How nearly that flow is laminar is of the utmost importance. In any wing it must be "somewhat" laminar or there wouldn't be flight -- and indeed if circumstances cause too much turbulence in that flow (usually meaning a small area of turbulence coming into existence at the trailing edge of the wing, then creeping forward up the top, and finally sweeping outward to involve a substantial portion of the top of the wing) then the wing stops creating lift. This is a "stall," and it has nothing to do with the engine, contrary to what the movies would lead one to believe. The conscious search for truly laminar wings seems to have begun with the P51 Mustang in WWII. But of course aerodynamic shaping and smoothness have always been issues. And something like laminar effects have always obtained: even in a Stearman (an open-cockpit, fabric-wing, early WWII biplane), if you have dust on the wing when you take off, you have the same dust on the wing when you land. Laminar flow and the difficulty of obtaining it can crop up in unexpected areas: There was a news account some years ago of a big dairy plant that had suffered contamination of a large volume of milk. They had miles of pipe, of course, which they cleaned assiduously, knowing that even a tiny amount of milk left between batches would go bad and could poison the next batch. Turned out after long investigation that there was a tiny area of micro-turbulence in the curve of one pipe -- a site where laminar flow had broken down and an almost-microscopic amount of milk had managed to lodge itself during cleaning. [post-comment]: Sigh. When trying to understand some physical phenomenon, it's often useful to assume conditions simpler than we find in the real world. No, there is no truly frictionless environment -- but in a "THEORETICALLY PERFECT laminar flow" etc. etc. I can't believe I had to say this.