M-shell

[ em-shel ]
/ ˈɛmˌʃɛl /

noun Physics.

the third shell of electrons surrounding the nucleus of an atom and containing, when filled, 18 electrons having principal quantum number three.

Origin of M-shell

First recorded in 1920–25
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019