The axial plane is an imaginary plane that divides a fold symmetrically into two equal halves. It is often used in structural geology to describe the orientation of folds and the relationship between different rock layers. The axial plane plays a crucial role in understanding the geometry of folded rock layers and the tectonic forces that caused the folding.
Reference: Ramsay, J. G. (1967). Folding and Fracturing of Rocks. McGraw-Hill.