Occurring in or representing two distinct forms.
Mid 19th century: from Greek dimorphos (from di- ‘twice’ + morphē ‘form’) + -ic.
“In this sexually dimorphic species, only the males have wings”
An antechamber, hall, or lobby next to the outer door of a building.
Early 17th century (denoting the space in front of the main entrance of a classical Roman or Greek building): from French, or from Latin vestibulum ‘entrance court’.