WordNet sense 1 ((usually plural) a necessary commodity for which demand is constant):

staple

WordNet sense 2 (reduced to the simplest and most significant form possible without loss of generality):

canonical, canonic

WordNet sense 3 (arising from or going to the root):

radical

WordNet sense 4 (being or involving basic facts or principles):

fundamental, underlying

WordNet sense 5 (of or being the essential or basic part):

elemental, primary

WordNet sense 6 (serving as a base or starting point):

introductory