morphological axis
noun
1 Crystallography (now rare, perhaps Obsolete ), the axis of rotational symmetry of a crystal system; the line of intersection of two planes of symmetry of a crystal system.
2(More generally) a line representing an axis of symmetry, or the longest linear dimension, in the shape of an object.
Origin
Late 19th century; earliest use found in Henry Palin Gurney (1847–1904), educationist.