National Advisory Committee for Aeronautics airfoils.
During the late 1920s and into the 1930s, the NACA developed a series of thoroughly tested airfoils and devised a numerical designation for each airfoil — a four digit number that represented the airfoil section’s critical geometric properties. By 1929, Langley had developed this system to the point where the numbering system was complemented by an airfoil cross-section, and the complete catalog of 78 airfoils appeared in the NACA’s annual report for 1933. Engineers could quickly see the peculiarities of each airfoil shape, and the numerical designator (“NACA 2415,” for instance) specified camber lines, maximum thickness, and special nose features. These figures and shapes transmitted the sort of information to engineers that allowed them to select specific airfoils for desired performance characteristics of specific aircraft.
SOURCE: Order of Magnitude: A History of the NACA and NASA, 1915–1990