LDS definition: In the premortal existence, Heavenly Father prepared a plan to enable us to become like Him and receive a fulness of joy. The scriptures refer to this plan as "the plan of salvation" (Alma 24:14; Moses 6:62), "the great plan of happiness" (Alma 42:8), "the plan of redemption" (Jacob 6:8; Alma 12:30), and "the plan of mercy" (Alma 42:15).
The plan of salvation is the fulness of the gospel. It includes the Creation, the Fall, the Atonement of Jesus Christ, and all the laws, ordinances, and doctrines of the gospel. Moral agency, the ability to choose and act for ourselves, is also essential in Heavenly Father's plan. Because of this plan, we can be perfected through the Atonement, receive a fulness of joy, and live forever in the presence of God. Our family relationships can last throughout the eternities.