1. The collection of all the electron states in an atom which have a given principal quantum number and a given orbital angular momentum quantum number.
2. The arrangement of electrons at various distances from the nucleus of an atom, according to the energy that they have.

Those with the least energy are in the shell closest to the nucleus, traditionally called the K shell, which can hold no more than two electrons.

The Q shell, farthest from the nucleus, can hold 98 electrons, but it is never completely filled.