Concise Bible Dictionary:

The learned are not agreed as to the derivation of the word shaddai and its signification: some giving it as “all bountiful,” others “all sufficient,” “all mighty” This is not at all surprising, for any name of God must be above mere human learning or definition, yet it was the ground of faith to those who had the revelation. The name first occurs in Genesis 17:1: God said to Abraham “I am the Almighty God.” This links it with the Patriarchs: it is the name by which God was known to them; and except to them, and in Job where it occurs very often, it is seldom found in the Old Testament The title “the Almighty” without the name of God being added, occurs first in Jacob’s address to his twelve sons before he died: the blessings upon Joseph were to be by “‘the Almighty,’.... blessings of heaven above, blessings of the deep that lieth under, blessings of the breasts and of the womb” (Genesis 49:25). Balaam uses the name in Numbers 24:4: Naomi also in her lamentations (Ruth 1:20-21). (See also Psa. 68:14; Psa. 91:1; Isa. 13:6; Ezek. 1:24; Ezek. 10:5; Joel 1:15).
In the New Testament the name Lord Almighty occurs in 2 Corinthians 6:18 in a quotation from Jeremiah, and a few times in the Revelation, but only once as “the Almighty” in Revelation 1:8: “I am Alpha and Omega, the beginning and the ending, saith the Lord, which is, and which was, and which is to come, the Almighty.” On the whole it is clear that the name was one of special relationship with the Patriarchs as that of Jehovah was with Israel. This is plainly declared: “I appeared unto Abraham, unto Isaac, and unto Jacob by the name of God Almighty, but by my name JEHOVAH was I not known to them” (Ex. 6:3). That of Father is now the revealed name of God by which Christians know Him, being brought by the work of Christ and through the operation of the Spirit into the relationship of children, and of sons. (See John 20:17; John 3:1; Gal. 4:4-5).
The name Almighty will appear again when God works out his purposes in power and judgment. It was revealed in connection with promises made in time, as Father is in connection with eternal counsels. The four living creatures cry day and night “Holy, holy, holy, Lord God Almighty, which was, and is, and is to come” (Rev. 4:8). (See also Revelation 21:22).