From Richard A Muller’s Dictionary of Latin and Greek Theological Terms:

“protoevangelium: literally, the protogospel; the first announcement of the redemption to be effected in and through Christ, given figuratively to Adam and Eve in the words of God to the serpent, “I will put enmity between you and the woman, and between your seed and her seed; he shall bruise your head, and you shall bruise his heel” (Gen. 3:15); in Reformed federalism, the inception of the covenant of grace.”