from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A diatomic molecule existing in an energy level above the ground state.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. Any diatomic molecule, at least one of whose atoms is in an excited state
exc(ited) + (d)imer.(American Heritage® Dictionary of the English Language, Fourth Edition)
exci- + -mer (Wiktionary)