"Excited atoms" refers to atoms that have absorbed energy and become more energized than their normal state. This extra energy causes the atoms to vibrate or move faster, producing light or heat. Full definition
The problem is that the sound passes through the rod only once, failing to make use of many excited atoms. (discovermagazine.com)
A source of energy excites the atoms in the gain medium,... (newscientist.com)
When an already excited atom is hit by another photon, however, it can't absorb it; instead it releases a photon of the same color, or frequency. (discovermagazine.com)