Definition of the generation of the Son
It is that eternal and necessary act of the first person in the Trinity, whereby He, within the divine Being, is the ground of a second personal subsistence like His own, and puts this second person in possession of the whole divine essence, without any division, alienation, or change.
Louis Berkhof, Systematic Theology (Grand Rapids, MI: Wm. B. Eerdmans publishing co., 1938), 94.