The most stable state of an atom is called the ground state. In the ground state, all the electrons are in the lowest energy levels, so their energy cannot be lowered further by moving into other orbitals. This stable state will remain until energy is applied from the outside.
The opposite of a ground-state atom is an excited-state atom. This refers to any state of the atom that has energy greater than the ground state. The total energy of the electrons in an excited-state atom can be lowered by moving electrons into a lower-energy orbital.
An atom does not usually stay in an excited state for long. An excited atom emits energy, such as photons, to reduce the overall atomic energy to a lower state. This process is called decay and is the opposite of excitation.