Belonging to the basic constituents from which everything else is made. Of what is not reducible further.
Latin elementum (a first principle, a rudiment); the adjective form naming what will not reduce any further.
Its root may be the alphabet itself: one account traces elementum to the letters L, M, N once recited together as a teaching aid, the first constituents a learner was ever given. Elemental keeps that same claim to be first, whatever the letters actually were.