What is the X in X-mas?
structural isomerism n.
Isomerism involving the same atoms in different arrangements.
isomerism i·som·er·ism (ī-sŏm'ə-rĭz'əm)
The phenomenon of the existence of isomers.
The complex of chemical and physical phenomena characteristic of or attributable to isomers.
The state or condition of being an isomer.