From the Introduction of NACA Report No. 460, The Characteristics of 78 Related Airfoil Sections ...:
Airfoil profiles may be considered as made up of certain thickness forms disposed about certain mean lines. The major shape variables then become two, the thickness form is of particular importance from a structural standpoint. On the other hand, the form of the mean line determines almost independently some of the most important aerodynamic properties of the airfoil section, e.g., the angle of zero lift and the pitching moment characteristics.
Very old stuff, but still valid. The 'mean line' is equidistant between top and bottom surfaces of the airfoil, and 'camber' describes its curvature, i.e. a symmetrical airfoil has zero camber.