cainite noun 1. One of the descendants of Cain, the first-born of Adam, according to the account in Genesis 2.A member of a Gnostic sect of the second century, who regarded the God of the Jews, the Demiurge of the Gnostic system, as an evil being, and venerated all who in the Old Testament record opposed him, as Cain, Korah, Dathan, Abiram, and the inhabitants of Sodom