The Feast has two different names. It’s the same feast, just with two different names. Some call it “The Dormition of the Theotokos” and some call it “The Assumption of the Blessed Virgin Mary.” Why the two names? Is there are difference? Is one better or more correct than the other?
The word “dormition” means “falling asleep.” Specifically, it denotes the death or passing away of the Holy Mother of God. With that word, then, we confess that the Virgin Mary truly died, as did her Divine Son; and then was raised from the dead as Christ raised the little girl when He said, “The child is not dead, but sleeping.”
The word “assumption” refers Mary being taken up, in her body, into heaven. This is similar to the ascension of her Divine Son. Although he ascended by His own power, the Holy Mother was aided in her going up—just as the prophet Elijah went up to heaven, not under his own power but in a ‘fiery chariot.’
These two words, then, focus on two different aspects of this gracious act of the Son for His Mother. She was raised from the dead (dormition), and she was raised in her body (assumption) to be seated “in heavenly places.” (See Ephesians 2.4-7)
In no instance does the Orthodox Church teach that the Blessed Virgin did not die. Like her Son, she tasted or experienced death. But also like Him, her body was glorified and transformed so that she might be with Him, at His side.
Our Lady did not endure an extended rest in the grave. According to St John of Damascus, after she fell asleep in the Lord, Mary’s body was buried in a coffin in Gethsemane
where for three days the singing of the angelic choirs persevered relentlessly. After the third day, those songs having ceased, the attending apostles opened the coffin at the request of Thomas, who was the only one who had been away from them, and who, on the third day, wanted to venerate the body that had [given birth to] God. But … they found only the funeral dresses put there, from which an ineffable perfume emanated that penetrated them, and they closed the coffin. Overwhelmed … here is the only thing they could conclude: the one who in his own person deigned to incarnate in her and become a man, God the Word, the Lord of glory, and who kept her mother’s virginity intact after his birth, had still wanted, after his departure from below, to honor this virgin and immaculate body with the privilege of incorruptibility; and with a translation prior to the common and universal resurrection.
Source
Therefore, the Lord honored His Mother by raising her on the third day, and then transporting her in her glorified body to heaven.
Yet, the Dormition or Assumption is not just about Mary. She enters heaven to show us that our mortal bodies, made of the same earth, can also be joined to heaven; that the lowest person can sit with God; that her prayer to be exalted by humility has been answered. For the woman was despised by her own kin, nearly disowned by her own spouse, and closeted by so many Christians. Yet in the Assumption, she exalted and literally lifted up to be what she always was: the best of all humanity.
-Fr John