According to MDBG.net, it means "joy" in Chinese. Most Chinese words are made up of two characters, but some characters can stand alone. I don't know how much this character is used by itself though.
Thanks thefabfive. Rep added. Does anyone know how popular 禧 is or how its used. From what I gather its not the most common word used for joy - I may be wrong though.
this term is not commonly used for joy, in fact me myself being a native, i hardly see that word being used for joy. the one that we would commonly use would be 喜. its not a keeper IMHO.