1the hard protective outer case of a mollusc or crustacean:cowrie shells [mass noun]:the technique of carving shell
the thin outer covering of an animal’s egg, which is hard and fragile in that of a bird but leathery in that of a reptile.
the outer case of a nut kernel or seed:peanuts roasted in their shells
the carapace of a tortoise, turtle, or terrapin.
the wing cases of a beetle.
the integument of an insect pupa or chrysalis.
(one's shell) used with reference to a state of shyness or introversion:she’ll soon come out of her shell with the right encouragement
2an explosive artillery projectile or bomb:the sound of the shell passing over, followed by the explosion [as modifier]:shell holes
a hollow metal or paper case used as a container for fireworks, explosives, or cartridges.
North American a cartridge.
3something resembling or likened to a shell because of its shape or its function as an outer case:pasta shellsbaked pastry shells filled with cheese
the walls of an unfinished or gutted building or other structure:the hotel was a shell, the roof having collapsed completely
an outer form without substance:he was a shell of the man he had been previously
4the metal framework of a vehicle body.
a light racing boat.
an inner or roughly made coffin.
the hand guard of a sword.
5 Physics each of a set of orbitals around the nucleus of an atom, occupied or able to be occupied by electrons of similar energies:in a multi-electron atom, the lowest energy shells fill up firstan electron descending from one shell to a lower one emits an X-ray