Even though its celebration is keyed to the beginning of Sukkot, Shmini Atzeret is a separate holiday. Because it is considered as separate from Sukkot, the blessing of "Shehecheyanu" is recited - the wife, when she lights the candles ushering in the Holiday and the husband, when he recites the Kiddush at the evening Seudah (festive meal).
"Shmini" means the eighth; in general, the number eight symbolizes perfection, as it does in the case of "brit milah," the covenant of circumcision. The brit is performed on the eighth day to "complete," as it were, or to instill the potential for perfection, in the human being.
"Atzeret" means "holding back," or that which is held back for the purpose of bringing it to its state of completion. Thus, the name "Shmini Atzeret" means the eighth day which is the additional day that brings the seven-day holiday of Sukkot to its state of perfection.